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Formation and Stability of Surface / Bulk Nanobubbles Produced by Decompression at Lower Gas Concentration
Abstract
Nanobubbles have many fascinating properties and the mechanism of their formation and stability still need further exploration. According to the conventional theory and experiences, it was suggested that surface/bulk nanobubbles would only be produced at a condition of high gas concentration and stabilized in a gas oversaturated state. However, we showed here that both surface and bulk nanobubbles could be formed at a condition of low gas concentration and exist under an unsaturated gas environment in water for a long time. In our experiments, the surface/bulk nanobubbles were produced by a new method of water decompression started from the normal pressure. Sufficient surface/bulk nanobubbles could be generated during the decompression within a certain time and were quite stable even after the pressure being recovered to the normal. The evolution process of the bulk nanobubbles with the time of decompression was studied and it was found that the concentration of the bulk nanobubbles was firstly increased and then decreased. In addition, the size of the bulk nanobubbles was increased during this decompression process. Our results revealed new information on the unique behavior of nanobubbles and should be helpful to understand their formation and stability mechanism as well as their applications.
... The mechanisms proposed in the last 20 years for BNB stability can be divided into two categories: the enrichment of interfacial charges (zeta potential) [21,22] and the adsorption of contaminants [23,24]. Although these models are partly capable of explaining why BNBs are stable, there are some phenomena that cannot be explained by these models: the stability of BNBs at low gas saturation or at zeta potentials close to 0 [25][26][27] and their almost constant size (∼100 nm) in different reports [26][27][28][29]. ...
... These studies help us to understand the properties of BNBs. However, the results of these studies still partly contrast with some experimental observations that BNBs remain stable for a long time at rather low gas saturations (S ∼ 0) [25] as a result of the exposure of nanobubble solutions to the ambient atmosphere. Experiments also show that, at least for certain cases [26] at low gas saturation, the required interfacial charge density and zeta potential for stabilizing BNBs [22] are out of reach. ...
... Here a reference state at a pressure P sat is chosen with the gas is saturated in solution, and g sat m and c g sat are the corresponding chemical potential and solubility of the dissolved gas. Therefore, for a nanobubble in a state of thermodynamic equilibrium, the equality of chemical potential for gas molecules requires » This is also consistent with the experimental observations that even at low gas supersaturation (S 0 » ) a large number of BNBs can exist stably [25]. ...
In our previous work [2022 Phys. Chem. Chem. Phys 24 9685], we show with molecular dynamics simulations that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces. This observation closely resembles the stability origin of microemulsions and inspires us to propose here that in certain cases, stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions: the nanobubble phase and the bubble-containing solution phase coexist with the external gas phase. The three-phase coexistence is then validated by molecular dynamics simulations. The stability mechanism for bulk nanobubbles is thus given: The formation of compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension, and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles. With the monolayer model, we further interpret several strange behaviors of bulk nanobubbles: the gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension, and the typical nanobubble size of 100nm can be explained through the small bending constant of the monolayer. Finally, through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can ubiquitously exist in aqueous solutions.
... It has also been reported that bulk nanobubbles can be produced in deionised water by decompression [53][54][55][56][57][58]. One of the typical experimental set-ups is shown in Fig. 3. Deionised water is pre-pressurised with air at different pressures for 30 min. ...
... If bulk nanobubbles indeed contain gas, their number density and/or size distribution would have a sensitive response to degassing. As described in the previous section, nanoentities were produced by mixing solvents (ethanol and deionised water) [71,73] and by decompression at lower gas concentrations [58]. In the former, nanoentities were detected and their concentrations in pure water, in ethanol, in a solution of degassed ethanol-water, and in natural ethanol-water mixtures were calculated. ...
... The results showed that the concentration of nanoparticles significantly decreased after ethanol and water were degassed (Fig. 13). Similar results were also obtained by Fang et al. [58] and recently by [73]. Therefore, they concluded that these nanoentities were gaseous domains. ...
The decreasing size of minerals that need to be separated with flotation technique has posed a rigorous challenge to conventional flotation techniques. It's urgent for the researchers working in the field of minerals processing to develop a kind of new, efficient but economic flotation technique. In the past dozens of years, a mass of reports regarding the application of bulk nanobubbles in flotation has been emerging and the positive effect of bulk nanobubbles has been widely verified by the flotation performance of various minerals. The rapid and brilliant progresses made in this field and the huge potential of application in minerals processing motivated the present work. In this work, we reviewed various techniques to produce bulk nanobubbles and the widely used measurement techniques for bulk nanobubbles and analyzed the disadvantages of them. These measurement techniques fail to identify the real identification of nanoentities, which have resulted in debates about the real identification of the observed nanoentities. Then, we reviewed the methods as in detail as possible that were proposed by many researchers to identify the identification of nanoentities, which should help close the relevant debate. Next, considering it that flotation is usually performed in such a liquid environment with complex physical and chemical properties (pH, salt, surfactant, ultrasonication et al.), we reviewed the responses of bulk nanobubbles to external stimuli including the factors described above. Only the bulk nanobubbles keep stable all the time during a flotation process does it make sense to employ bulk nanobubbles for flotation. In the last section, we reviewed the underlying mechanism of how the flotation performance was improved in the presence of bulk nanobubbles. The aim of this review is to provide researchers who are working in the area of mineral processing with rough information about nanobubbles from the field of interface physics and shed some light on developing new techniques for fine particle separation.
... Acoustical cavitation has been employed to prepare NBs in aqueous solutions. [16][17][18]31 Under ultrasonic irradiation, the generation and reduction of NBs in liquids occurred simultaneously, and the bubble core grows into cavitation bubbles through the expansion and compression of NBs. Cavitation bubbles break into tiny bubbles of various sizes, such as bubble nuclei and NBs. ...
... A degassing process has been widely used as a simple way to verify the formation of NBs in solutions. 19,26,31 Since propanol can partly evaporate when exposed to low pressure for a long time, the concentration of nanoparticles detected by the NTA should increase if there are nano-impurities in the solution. In this experiment, we also conducted degassing experiments to figure out whether the nanoparticles detected by the NTA were indeed NBs. ...
... Therefore, there would be no air NBs formed during ultrasonic treatment without a gas−liquid interface. 31,60 In our experiment, 20 mL glass bottles were fully filled with propanol solution, therewith sealed with a lid to exclude the existence of a gas−liquid interface. As shown in Figure 2e, the bubble number density remained basically unchanged in such a case. ...
Bulk nanobubbles (NBs) have attracted wide attention due to their peculiar physicochemical properties and great potential in applications in various fields. However, so far there are no reports on bulk NBs generated in pure organic systems, which we think is very important as NBs would largely improve the efficiency of gas–liquid mass transfer and facilitate chemical reactions to take place. In this paper, we verified that air and N2 NBs could be generated in a series of alcohol solutions by using various methods including acoustical cavitation, pressurization–depressurization, and vibration. The experiments proved that NBs existed in alcohol solutions, with a highest density of 5.8 × 10⁷ bubble/mL in propanol. Our results also indicated that bulk NBs could stably exist for at least hours in alcohol systems. The parameters in generating NBs in alcohols were optimized. Our findings open up an opportunity for improving gas–liquid mass transfer efficiency in the field of the chemical industry.
... The goal of preparing nanobubbles via the depressurization method is to alter gas solubility by controlling the pressure exerted on the liquid within the conta When the pressure is reduced, the gas solubility decreases, causing precipitation and formation of nanobubbles [36]. Fang et al. [45] dripped unsaturated pure water o HOPG and performed a five-minute depressurization test, which resulted in the de tion of the nanobubbles through AFM. This result indicates that a short-term reduc in pressure locally saturates the gas concentration on the surface of the HOPG and erates nanobubbles. ...
... When the pressure is reduced, the gas solubility decreases, causing precipitation and the formation of nanobubbles [36]. Fang et al. [45] dripped unsaturated pure water onto HOPG and performed a five-minute depressurization test, which resulted in the detection of the nanobubbles through AFM. This result indicates that a short-term reduction in pressure locally saturates the gas concentration on the surface of the HOPG and generates nanobubbles. ...
Nanobubbles represent a special colloidal system, as they have high stability and large specific surface areas. The preparation of nanobubbles is currently a hot research topic, as it crucial to investigate their characteristics and expand their applications. This article explains the mechanism of generating nanobubbles based on chemical and physical methods, introduces their basic composition’s structure and properties, summarizes the methods of preparing bulk nanobubbles (BNBs) and surface nanobubbles (SNBs), and clarifies the preparation principles and techniques. Seven practical applications of nanobubbles are cited in this paper, including their use as ultrasonic contrast agents in medical imaging, drug delivery systems in drug transportation, promoters of plant growth by affecting plant respiration and water absorption at the roots, tools to remove dirt from surfaces by generating energy during nanobubble bursting, producers of high-density negative ions and free radicals to react with pollutants in wastewater, tools to reduce the resistance of the fluid flow through channels by lowering the internal friction, and means of improving the mineral flotation recovery rate by enhancing the absorption capacity of bubbles to minerals. Finally, the future development of nanobubble preparation technology is discussed, including their roles in optimizing equipment and preparation methods; improving the quantity, efficiency, stability, controllability, and homogeneity of nanobubble generation; and promoting the industrial production of nanobubbles.
... In the past 20 years, the proposed mechanisms for interpreting bulk nanobubble's stability are mainly divided into two categories: the enrichment of interfacial charges (zeta potential) 21,22 and the adsorption of contaminants. 23,24 Although these models are partly capable of interpreting why bulk nanobubbles are stable, there are some phenomena that cannot be explained by these models: the stability of bulk nanobubbles at low gas saturation or at zeta potential close to 0, [25][26][27] and their almost constant size (~ 100 nm) from different reports. [26][27][28][29] In detail, according to these models the stable radius of nanobubbles always depends on the number of charges and that of contaminants enriched on the nanobubble interface. ...
... [26][27][28][29] For both the zeta potential mechanism and the contaminant mechanism, 23 they require the dissolved gas to have a certain level of gas supersaturation, which provides an additional pressure (Laplace pressure) to balance the surface tension of the bubble interface. This is not enough to explain experimental observations that the stable bulk nanobubbles can be observed at rather low gas saturation (~0 ), 25 which is common condition in practice due to the exposure of nanobubble solutions to the ambient atmosphere. These disagreements demonstrate the existence of some important characteristics unidentified for the stabilization mechanism of real nanobubbles. ...
In our previous work [Phys. Chem. Chem. Phys. 2022, 24, 9685], we show with molecular dynamics simulations that bulk nanobubbles can be stabilized by forming a compressed amphiphile monolayer at bubble interfaces. This observation closely resembles the stability origin of microemulsions and inspires us to propose here that stable bulk nanobubbles can be regarded as gaseous analogues of microemulsions: the gas-in-water nanobubble phase coexisting with the external gas phase. The stability mechanism for bulk nanobubbles is then given: The formation of compressed amphiphilic monolayer because of microbubble shrinking leads to a vanishing surface tension, and consequently the curvature energy of the monolayer dominates the thermodynamic stability of bulk nanobubbles. With the monolayer model, we further interpret several strange behaviors of bulk nanobubbles: the gas supersaturation is not a prerequisite for nanobubble stability because of the vanishing surface tension, and the typical nanobubble size of 100nm is due to the small bending constant of the monolayer. Finally, through analyzing the compressed amphiphile monolayer model we propose that bulk nanobubbles can ubiquitously exist in aqueous solutions.
... Alternatively, these ions could stabilize preexisting nanoscale gas bubbles against dissolution (Akulichev, 1966) producing bubbstons (Bunkin and Bunkin, 1992;Sankin and Teslenko, 2003). Ion stabilization likely explains the observed longevity of bulk nanobubbles (Nirmalkar et al., 2018b,a;Fang et al., 2018;Zhu et al., 2016;Uchida et al., 2016) and suggests that acoustic methods could be measuring the onset of heterogeneous cavitation in a subpopulation of ion-stabilized, nanoscale nuclei rather than a genuine homogeneous threshold (Maxwell et al., 2013;Sankin and Teslenko, 2003). Nevertheless, the reproducibility of acoustic threshold measurements in water of variable purity implies that this subpopulation of nuclei is highly consistent (Maxwell et al., 2013;Borkent et al., 2007;Ando et al., 2012), ubiquitous in water (Azouzi et al., 2013;Davitt et al., 2010), and intrinsic to water and water-based tissues (Bader et al., 2019;Bunkin and Bunkin, 1992). ...
... Prior work in heterogeneous cavitation suggests that nucleus sizes follow a lognormal (Ben-Yosef et al., 1975;Ando et al., 2011) or Weibull (Wienken et al., 2006) distribution, but it is not clear that these distributions are applicable to nanoscale nuclei present at threshold. While it is possible to measure the size distributions and other characteristics of nanobubbles (Nirmalkar et al., 2018b,a;Fang et al., 2018;Zhu et al., 2016;Uchida et al., 2016;Bunkin et al., 2014Bunkin et al., , 2016Jin et al., 2007), such studies involve methods that nucleate multiple bubbles simultaneously in water that often contains added ions (Zhu et al., 2016;Uchida et al., 2016;Bunkin et al., 2014Bunkin et al., , 2016Jin et al., 2007). Though they are likely stabilized by similar physics (Akulichev, 1966), these nanobubbles are not necessarily representative of the hypothesized nanoscale nuclei present at the acoustic cavitation threshold in deionized water. ...
Understanding the acoustic cavitation threshold is essential for minimizing cavitation bioeffects in diagnostic ultrasound and for controlling cavitation--mediated tissue ablation in focused ultrasound procedures. The homogeneous cavitation threshold is an intrinsic material property of recognized importance to a variety of applications requiring cavitation control. However, acoustic measurements of the cavitation threshold in water differ from those predicted by classical nucleation theories. This persistent discrepancy is explained by combining novel methods for acoustically nucleating single bubbles at threshold with numerical modeling to obtain a nucleus size distribution consistent with first--principles estimates for ion--stabilized nucleii. We identify acoustic cavitation at threshold as a reproducible subtype of heterogeneous cavitation with a characteristic nucleus size distribution. Knowledge of the nucleus size distribution could inspire new approaches for achieving cavitation control in water, tissue, and a variety of other media.
... The experimental proof that bulk nanobubbles exist is also less than definitive. Dynamic light scattering (DLS) experiments [3] consistently claim that bulk nanobubbles have radii that (i) typically range from ∼50 to 500 nm [8,[34][35][36], (ii) appear to be strictly bounded to ≲1 μm, and (iii) increase with ionic concentration [37][38][39][40][41]. However, sceptics do not regard DLS characterization as authoritative proof of the existence of bulk nanobubbles, since it depends on processing light-scattered speckle that cannot be unambiguously attributed to compressible bubbles. ...
... Oversaturation ζ > 0 is required for nanobubbles to nucleate, through mechanical aeration of the water [10] or water-ethanol exchange [66,67], ζ ∼ 1-3 being generally reported. However, we also note that nanobubbles generated through external energy input in an environment without substantial oversaturation (e.g., by pressure variations [36,68] or electrochemistry [8]) can exist in stable equilibrium, too. Once nucleated, these bubbles are stable in exactly saturated (ζ ¼ 0) liquids. ...
The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below 10 μm. Our calculations recover three persistently reported—but disputed—properties of bulk nanobubbles: that they stabilize at a typical radius of ∼100 nm, that this radius is bounded below 1 μm, and that it increases with ionic concentration.
... Decompression: Controlled decompression processes that manipulate pressure within a liquid medium induce gas molecules to congregate into nanoscale bubbles, thereby presenting a method for nanobubble formation [34]. • Solvent Substitution: Specific alterations in the chemical properties of solvents can induce the spontaneous formation of nanobubbles under controlled conditions. ...
Nanobubble technology has emerged as a transformative approach in bioprocessing, significantly enhancing mass-transfer efficiency for effective microbial activity. Characterized by their nanometric size and high internal pressure, nanobubbles possess distinct properties such as prolonged stability and minimal rise velocities, allowing them to remain suspended in liquid media for extended periods. These features are particularly beneficial in bioprocesses involving aerobic strains, where they help overcome common obstacles, such as increased culture viscosity and diffusion limitations, that traditionally impede efficient mass transfer. For instance, in an experimental setup, nanobubble aeration achieved 10% higher soluble chemical oxygen demand (sCOD) removal compared to traditional aeration methods. Additionally, nanobubble-aerated systems demonstrated a 55.03% increase in caproic acid concentration when supplemented with air nanobubble water, reaching up to 15.10 g/L. These results underscore the potential of nanobubble technology for optimizing bioprocess efficiency and sustainability. This review delineates the important role of the mass-transfer coefficient (kL) in evaluating these interactions and underscores the significance of nanobubbles in improving bioprocess efficiency. The integration of nanobubble technology in bio-processing not only improves gas exchange and substrate utilization but also bolsters microbial growth and metabolic performance. The potential of nanobubble technology to improve the mass-transfer efficiency in biotechnological applications is supported by emerging research. However, to fully leverage these benefits, it is essential to conduct further empirical studies to specifically assess their impacts on bioprocess efficacy and scalability. Such research will provide the necessary data to validate the practical applications of nanobubbles and identify any limitations that need to be addressed in industrial settings.
... Such contaminants, once formed, can be stable in the system for quite a long time compared to particle-scale contaminants. [35][36][37][38][39][40][41][42] and potentially drastically alter the flow behavior. [43][44][45] Here, we show that the flow behavior of such laboratory-made oil-based suspensions is strongly affected by the absolute humidity of the ambient air. ...
Immiscible contaminants are commonly involved in naturally occurring suspensions. The resulting variations of their flow behavior has rarely been evaluated. Here, we investigate the variation of the viscosity of the oil-based two-phase suspension over a period of two years, which is exposed to the ambient air at the production stage. We find that the air's absolute humidity, which strongly varies with the seasons, causes exchanges of water droplets with the suspension, substantially altering its shear-thinning behavior. Only in winter, when the humidity is low, is the latter close to that of ideal two-phase suspensions. Our measurements suggest that, when the surface roughness of the suspended solid particles is sufficiently low, immersed droplets remain in a free state, effectively increasing repulsion between particles, weakening shear thinning. In contrast, when the roughness is sufficiently high, immersed droplets become trapped on the particle surfaces, inducing an attractive particle interaction via water bridging, enhancing shear thinning.
... In the case of surface nanobubbles the following generation methods have been reported: solvent exchange (Fang, 2018;Millare and Basilia, 2018), electrochemical reactions (Gadea et al., 2020;Suvira and Zhang, 2021), immersion of hydrophobic substrate into water with temperature increase or pressure decrease . However, the scientific interest is primarily focused on bulk NBs generation techniques. ...
Nanobubble technology is an emerged solution to address climate change, environmental challenges, cost and energy reduction in industrial processes, optimization of therapeutic and diagnostic techniques and other applications. Although nanobubble production and exploitation is a recently developed field, there are numerous reports and studies of their properties and promising implementation in various sectors. This work aims to give a condense information regarding the most recent (since 2017) scientific findings in the potentials of nanobubbles as a versatile and sustainable technology. Environmental, agricultural, medical/bio-medical and other applications are reviewed and the most indicative of each sector is presented in detail. A special focus is given on water and wastewater treatment implementation.
... Degassed water was also prepared by the thawing-degassing cycle. 62 The ice stored in a glass bottle was placed in a vacuum device (DZF-6210, Shanghai Jing Hong Laboratory Instrument Co., Ltd China) for degassing and thawing for 10 hours, and then refreezed in a freezing chamber. The regular degassed water experienced more than three cycles. ...
Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.
... The existence of nanobubbles has already been confirmed but the mechanism of their extraordinary stability remains unclear since the publication of first nanobubble images in 2000 (Ishida et al., 2000;Lou et al., 2000). This Research Topic intends to collect the latest advances in explanation on the stability of surface and bulk nanobubbles and provide the latest perspectives in terms of theoretical explanations by considering some important factors such as surface tension, surface chemical heterogeneity and gas saturation, etc (Fang et al., 2018;Zhou et al., 2020). Pan et al. investigated the influence of the variable surface tensions on the stability of surface nanobubbles. ...
... Because of the low number density of the bulk nanobubble suspensions, as given by the zeroth moment, n(R, t)N 0 (t) = ∫ n(R, t)dR, which is less than the order of 10 10 bubbles per milliliter, as determined by nanoparticle tracking analysis. [17][18][19]21 The average distance between two neighboring bubbles is much larger than the radius of the bulk nanobubbles. In this dilute limit, we can treat the influence of all other bubbles as an effective gas saturation level f(t). ...
... This method can produce a high concentration of NBs in large scale and has been introduced as a method of industrial generation of NBs by Iwaki Co. Ltd., Japan and by AS ONE Corporation, Osaka, Japan.Alternatively, the pressurized gas-liquid mixing step can be omitted by simply starting from an unsaturated gas-aqueous solution at normal temperature and pressure. Upon lowering the pressure below atmospheric conditions, the solution eventually becomes a supersaturated gas solution, and extra gas molecules might be released and aggregate into bubbles.165 With this method it was found that the concentration of the NBs first increased and then decreased as a function of the decompression time, while the size of the NBs increased during this decompression process. ...
Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.
... The decompression time and the pH of the solution are found to affect the concentration of produced BNBs [60]. Unlike previous beliefs that nanobubble could only appear in a gas oversaturated condition, it was proposed that BNBs could be generated upon decompression and stay in an unsaturated gas environment for a long time [43]. Some studies also revealed that the bubble sizes could be adjustable by periodic pressure change [50,51] or pressurized through the porous membrane [48], and the efficiency of BNBs production could be improved by introducing some additives like graphene oxide [44], nanoparticles [61], and organic impurities [62,63]. ...
Bulk nanobubbles (BNBs) are submicron gaseous domains dispersed in solutions which are supposed to survive for several hours or even days. In recent years, there has been a rapid growth on the research and extraordinary applications of BNBs. However, conventional theories based on gas diffusion and Laplace pressure predicted that nanoscale gas bubbles in water should dissolve within microseconds, presenting a modern-day paradox in current nanobubbles researches. Also, it is still challenging to efficiently produce BNBs and determine their gaseous nature with the available techniques. In this review, we start from a general introduction and brief history of nanobubbles researches and revisit the current progress on the generation methods and detection techniques. Two possible formation mechanisms are suggested and the plausibility of the proposed theories on BNBs stability is discussed with some suggestions for future studies on bulk nanobubbles.
... The parameters used vary greatly between different reports, and also the sensitivity of the investigated bubbles. To complicate matters, depressurization has also been reported to generate nanobubbles, on dispersed particles as well as in bulk, rather than destroy them [45,46,47]. Bubbles stabilized by unknown adsorbed material in sea water [43] were sensitive to comparably small pressure changes compared to coated bubbles in a commercial contrast agent [48]. ...
History has shown that it is not as easy as one might think to differentiate between bulk nanobubbles and nanodroplets or nanoparticles. It is generally easy to detect colloids (i.e. something that looks different and e.g. scatters light differently than its surrounding solvent), but less easy to determine the nature of these colloids. This has led to misinterpretations in the literature, where nanodroplets or nanoparticles have mistakenly been assumed to be nanobubbles. In this paper we review a multitude of experimental methods and approaches to prove the existence of bulk nanobubbles. We conclude that combinations of optical detection with physical perturbations such as pressure or ultrasound, or phase sensitive holographic methods are the most promising and convenient approaches.
... Bulk nanobubbles have been recently proved to exist and get stabilized over long period of times [14][15][16]. Furthermore, there are experimental evidences to stabilize bulk nanobubbles on surfaces, thus creating stable surface nanobubbles, shed light on the development of a new class of contrast agents able to leverage the stability of these nanosized entities to generate sustained echogenic signals. ...
Nanoparticles able to promote inertial cavitation when exposed to focused ultrasound have recently gained much attention due to their vast range of possible applications in the biomedical field, such as enhancing drug penetration in tumor or supporting ultrasound contrast imaging. Due to their nanometric size, these contrast agents could penetrate through the endothelial cells of the vasculature to target tissues, thus enabling higher imaging resolutions than commercial gas-filled microbubbles. Herein, Zinc Oxide NanoCrystals (ZnO NCs), opportunely functionalized with amino-propyl groups, are developed as novel nanoscale contrast agents that are able, for the first time, to induce a repeatedly and over-time sustained inertial cavitation as well as ultrasound contrast imaging. The mechanism behind this phenomenon is investigated, revealing that re-adsorption of air gas nanobubbles on the nanocrystal surface is the key factor for this re-chargeable cavitation. Moreover, inertial cavitation and significant echographic signals are obtained at physiologically relevant ultrasound conditions (MI<1.9), showing great potential for low side-effects in in-vivo applications of the novel nanoscale agent from diagnostic imaging to gas-generating theranostic nanoplatforms and to drug delivery.
... With its high stability and high oxygen-transfer efficiency (Tyrrell and Attard, 2001;Fang et al., 2018), NB has significant potential to be applied in the oxygenation and in situ control of internal-nutrient release at the SWI in lakes (Zhang et al., 2018;Yu et al., 2019). In this study, oxygen NB modified mineral (ONBMM) was developed and applied to conduct an experimental study on the oxygen enrichment and in situ control of N and P pollutants at the SWI. ...
... The use of advanced probe techniques would certainly deepen our understanding of nanobubbles. To deal with this open question, an impressive amount of experimental work on surface and bulk nanobubbles has been conducted by Hu and Zhang and their coauthors in the past [7,15,55,[83][84][85]. As a complementation of the experimental exploration, theoretical and computational studies on surface and bulk nanobubbles emerged recently [86][87][88][89][90][91][92]. ...
... Now the initial BNBs was generated. Because nanobubbles size that depend on the pressure reduce under pressurization 4,50,51 , in second cycle, the gas dissolution in nanobubbles tends to occur with the increase of pressure again, and the dissolved nanobubbles would grow again as the pressure decreases. This process of the periodic pressure change is similar to the response of surface nanobubbles to acoustic field 52 . ...
Recently, bulk nanobubbles have attracted intensive attention due to the unique physicochemical properties and important potential applications in various fields. In this study, periodic pressure change was introduced to generate bulk nanobubbles. N2 nanobubbles with bimodal distribution and excellent stabilization were fabricated in nitrogen-saturated water solution. O2 and CO2 nanobubbles have also been created using this method and both have good stability. The influence of the action time of periodic pressure change on the generated N2 nanobubbles size was studied. It was interestingly found that, the size of the formed nanobubbles decreases with the increase of action time under constant frequency, which could be explained by the difference in the shrinkage and growth rate under different pressure conditions, thereby size-adjustable nanobubbles can be formed by regulating operating time. This study might provide valuable methodology for further investigations about properties and performances of bulk nanobubbles.
... Because of prominent advantages, such as a long stable time and high mass transfer efficiency (Tyrrell et al., 2001;Shi et al., 2018;Fang et al., 2018), NBs have a vast application potential from the perspective of lake SWI aeration and internal P control (Zhang et al., 2018). This study utilized a natural mineral (muscovite) as a base for the loading of oxygen nano-bubbles to develop a specialized novel material and technique for aeration of the SWI. ...
Due to the limited aeration capacity of current aeration techniques at the sediment-water interface (SWI), we developed a specialized aeration material aimed at the SWI, known as oxygen nano-bubble-modified minerals (ONBMMs). Furthermore, we simulated its aeration efficiency at the SWI and the control effects of internal phosphorous (P) release under anaerobic conditions during 20 days. High resolution diffusive gradients in thin films (DGT) and Planar luminescent optode (PO) technologies were used to measure the temporal variation of reactive P, reactive Fe (II) and dissolved oxygen (DO) of the SWI. These results show that ONBMMs can effectively increase the content of DO at the SWI and decrease the release flux of internal P from sediments. The use of ONBMMs reduced 97.9% of the soluble reactive P concentration of the overlaying water and reduced the release flux of DGT-P from sediments by 78.9%. Inhibition of reductive dissolution of Fe–P from sediments was the primary principle that effectively inhibited the input of internal P by ONBMMs. Therefore, ONBMMs are potentially promising technology for the treatment of internal P pollution in eutrophic lakes.
... 10 We note that a recently published manuscript reports that surface nanobubbles produced by decompression were stable in undersaturated water, which is consistent with our observations reported here. 58 ...
Surface nanobubbles should not be stable for more than a few milliseconds, however they have been shown to persist for days. Pinning of the three-phase contact line of surface nanobubbles has been proposed to explain the discrepancy between the theoretical and experimental results. According to this model, two factors stabilize surface nanobubbles, namely solution over-saturation and surface pinning. Hereby, we investigate experimentally the impact of the solution saturation on the stability of nanobubbles. For this purpose, surface nanobubbles have been nucleated on hydrophobic surfaces, by two methods, and then characterized by Atomic Force Microscopy (AFM). Thereafter, the surrounding liquid has been exchanged multiple times with partially degassed water. Two degassing techniques are presented. Both sets of experiments lead to the conclusion that surface nanobubbles are stable in under-saturated conditions for hours. We compare the measured lifetime of nanobubbles to calculations for pinned nanobubbles in under-saturated conditions. The stability of surface nanobubbles in undersaturated solutions observed here is incommensurate with the pinning mechanism as the origin of the long-term stability of surface nanobubbles.
Nanobubbles (NBs) have recently been used for fine particle flotation, and mixed cationic/anionic collectors have shown powerful effects in flotation of lepidolite. In this paper, the flotation tests, nano particle size analyses, contact angle tests, settling tests and high-speed camera observation was used to study the interaction between NBs and fine lepidolite particles with mixed cationic/anionic collectors. The results showed that the concentration of surface NBs and the presence of bulk NBs play an indispensable role in the promotion of lepidolite flotation by NBs. The size of NBs was larger near the isoelectric point and NBs were more stable by co-interaction with the mixed cationic/anionic collectors. The mechanism of the influence of NBs on the surface hydrophobicity of lepidolite was proposed, and the unstable effect of NBs on the flotation of fine-grained lepidolite was explained.
Hydrate-based technologies have excellent application potential in gas separation, gas storage, transportation, and seawater desalination, etc. However, the long induction time and the slow formation rate are critical factors affecting the application of hydrate-based technologies. Micro-nano bubbles (MNBs) can dramatically increase the formation rate of hydrates owing to their advantages of providing more nucleation sites, enhancing mass transfer, and increasing the gas–liquid interface and gas solubility. Initially, the review examines key performance MNBs on hydrate formation and dissociation processes. Specifically, a qualitative and quantitative assembly of the formation and residence characteristics of MNBs during hydrate dissociation is conducted. A review of the MNB characterization techniques to identify bubble size, rising velocity, and bubble stability is also included. Moreover, the advantages of MNBs in reinforcing hydrate formation and their internal relationship with the memory effect are summarized. Finally, combining with the current MNBs to reinforce hydrate formation technology, a new technology of gas hydrate formation by MNBs combined with ultrasound is proposed. It is anticipated that the use of MNBs could be a promising sustainable and low-cost hydrate-based technology.
Air nanobubbles (A-NBs) in a circulating cooling water system have not been investigated, although their role is significant. In this paper, the influences of the contents of main salts and other parameters on the physicochemical characteristics and scale inhibition performance of A-NBs in circulating cooling water were investigated and the scale inhibition mechanism of A-NBs in a simulated circulating cooling water system was explored. A-NBs realized a higher scale inhibition rate of 90%, which was higher than that of 1-hydroxyethane-1,1-diphosphonic acid (40%), and A-NBs stably existed for more than 5 days in the complex water environment. Four interface functions were proposed to interpret the scale inhibition effect of A-NBs in circulating cooling water as follows. First, the negatively charged surface of A-NBs adsorbed cations (Ca2+) reduced the concentration of scaling ions. Second, the negatively charged surface of A-NBs could also adsorb microcrystals, and their crystal-like seed action was conducive to the formation of large-size crystals, broke the rules of crystal growth, and reduced the adhesion of scales to the pipe wall. Third, A-NBs could also form a bubble layer after they were adsorbed on the inner surface of pipes, thereby preventing the deposition of scales on the surface. Fourth, A-NB burst caused local turbulence, increased the shear force onto the pipe surface, and reduced the scales adhering to the pipe surface. The interface effect of A-NBs in metal pipes is important in many industrial applications. This study laid the basis for the development of a new green A-NB scale-inhibiting technology.
Immiscible contaminants are commonly involved in naturally occurring suspensions. The resulting variations of their flow behavior has rarely been evaluated. Here, we investigate the variation of the viscosity of the oil-based two-phase suspension over a period of two years, which is exposed to the ambient air at the production stage. We find that the air's absolute humidity, which strongly varies with the seasons, causes exchanges of water droplets with the suspension, substantially altering its shear-thinning behavior. Only in winter, when the humidity is low, is the latter close to that of ideal two-phase suspensions. Our measurements suggest that, when the surface roughness of the suspended solid particles is sufficiently low, immersed droplets remain in a free state, effectively increasing repulsion between particles, weakening shear thinning. In contrast, when the roughness is sufficiently high, immersed droplets become trapped on the particle surfaces, inducing an attractive particle interaction via water bridging, enhancing shear thinning.
Nanoscale gas bubbles are paid more and more attention due to their significant applications in different fields including the environmental remediation, plant and animal growth as well as medical diagnosis, etc. As reported, the local gas saturation plays an important role for the formation of surface nanobubbles (NBs) but is less importance for their stability. As for bulk NBs, few researches focused on the influence of dissolved gas on their generation and stability because it is thought generally that a limited amount of gas could dissolve into water. Herein, we reported for the first time the relationship of dissolved gases (Kr, O2 and N2) and the formation and stability of bulk NBs. Firstly, we developed a compression-decompression method to produce the water with super-high concentration of dissolved gas. About 60 mg/L oxygen dissolved gas was created to promote the formation of bulk NBs. It was showed that high bulk NB concentrations were produced using the compression-decompression method by controlling the loading pressure and time at the same time. The evolution process showed that the concentration of dissolved gas would decrease quickly with the deposited time. However, the concentration of formed bulk NBs did not follow the same way as dissolved gas concentration. It exhibited a complicated change over the time. Typically, first sharp increase to one order higher concentration than at the beginning and then decreased with a fluctuation within 72 hours. More interestingly, the time of this sharp increase in nanobubble concentration depended on the type of gas, the krypton (Kr) gas system took longer time to reach the highest concentration and the oxygen (O2) as well as nitrogen (N2) gas system reached the highest concentration at about 4 h generally. The change of zeta potential of those NBs followed the same fluctuation as their concentration. Finally, we presumed a theoretical model to explain the evolution mechanism of bulk NBs. It indicated that there is a competition of different bubble behaviors (nucleation, clustering and coalescence) in different time periods. This study provides a new technique to produce high concentration of bulk NBs and dissolved gas in solution. Those results are very significance for further understanding the mechanism of formation and stability of bulk NBs under a super-high concentration of dissolved gas and may be used in some chemical reactions related with gas to promote the reaction efficiency.
The safety threat posed by Perfluoroalkyl acids (PFAAs) in drinking water is a growing concern. In this study, we loaded chitosan (CS) on granular activated carbon (GAC) to adsorb PFAAs, and we explored the role of nanobubbles in the adsorption process through experiments and density functional theory (DFT) calculations. Compared with GAC, we found that the use of the composite adsorbent (CS/GAC) enhanced the removal rate of perfluorooctanoic acid by 136% with the assistance of nanobubbles. PFAAs with different chain lengths have different adsorption mechanisms owing to surface activity differences. PFAAs with longer C–F chains can be directly enriched with amino groups on the CS or air–water interface on composite adsorbents. Additionally, PFAAs can be enriched with nanobubbles in solution to form nanobubble–PFAA colloids, which are adsorbed by protonated amino groups on CS through electrostatic interactions. We found that PFAAs with shorter C–F chains are less affected by nanobubbles, and DFT calculations indicated that the adsorption of short-chain PFAAs is mainly affected by electrostatic interactions. We also proved that the electrostatic interactions between CS and PFAAs are mainly derived from the abundant protonated amino groups.
Micro-nano bubbles (MNBs) have attracted extensive attention in recent years due to their distinctive features and physiological activities. The particle size and size distribution of MNBs are important properties of their application. This study presented a green method, namely magneto internal heat bubble generation (MIHBG), for preparing magnetic MNBs based on the magnetically internal heating technology. The effects of the operation conditions on the properties of the generated MNBs were discussed. The results showed that the MNBs with good uniformity could be successfully prepared via MIHBG. Under the preferred conditions, the average size of the MNBs was 341.5 ± 18.2 nm with the polydispersity index (PDI) of 0.240 ± 0.027 and the zeta potential of -42.4 ± 4.2 mV, respectively. The concentration of the generated MNBs could reach 1.5×10⁹ particles/mL. Transmission electron microscopy (TEM) image demonstrated that the MNBs had a gas core with the superparamagnetic iron oxide nanoparticles (SPIONs) adsorbed on the surface. The mechanism of the MNBs generation process was explained by the classical nucleation theory. The good uniformity of the magnetic MNB samples benefited from the uniform hotspots formed by the SPIONs via the alternating magnetic field. In addition, the lipid-encapsulated Xenon bubbles (Xe-LBs) were prepared based on the MIHBG process to improve the stability of MNBs for the determination of protecting cells from intermittent hypoxic damage. MTT assay and TUNEL experiments indicated that the Xe-LBs played a significant role in protecting cells from intermittent hypoxic damage and inhibiting apoptosis of cells. These results revealed that the MIHBG process provided a novel way to prepare MNBs for potential applications such as protecting hypoxic neurons.
The effects of liquid temperature on bulk nanobubbles (BNBs) generation using ultrasound was investigated. When liquid at different temperatures and relatively similar dissolved oxygen concentration (25 °C: 8.29 ppm; 50 °C: 8.11 ppm; and 75 °C: 7.78 ppm) were sonicated, a high number of negatively charged (−13 to −21 mV) BNBs were consistently generated within a few minutes. As the temperature of the liquid increased, it is expected that the concentration of BNBs considerably increased due to a lot of nucleation site and strong collapse of tiny bubbles. On the other hand, in a liquid with relatively low temperature, a high number of large bubbles were formed and the BNBs concentration considerably decreased. It is suggested that it has less nucleation sites compared to a liquid with higher temperature; thus, the bubble growth was accelerated by rectified diffusion and coalescence, owing to less nucleation sites versus dissolved gas concentration.
The interplay of food proteins and macro or microscopic bubbles at the air-water interface that shaped the ultimate structures of proteins is well-known, yet knowledge was blank on the interactions between food proteins and nanoscopic bubbles (i.e., nanobubbles). In this study, bulk air nanobubbles were successfully fabricated in the aqueous solutions following a facile “cyclic compression-expansion” method. Subsequently, the effects of generated air nanobubbles and various factors including pH, protein concentration, nanobubble concentration, and ionic strength on the self-assembly behavior of 7S globulins isolated from pea proteins were investigated. It was revealed that air nanobubbles acted as soft templates to trigger 7S globulins self-assembly into core-shell nanospheres adjacent to the protein isoelectric point (∼pH 5). An enhancement of ionic strength from 0 to 0.4 mol L⁻¹ led to increased particle size, whereas attenuating interactions between 7S globulins and air nanobubbles. The particle size of nanoparticles was also demonstrated to be protein and nanobubble concentration-dependent. Protein secondary structures were modified by air nanobubbles with apparently increased contents in random coils and decreased contents in α-helix. Changes in protein tertiary structures demonstrated that 7S globulins are exposed to a more hydrophobic microenvironment with reduced surface hydrophobicity, after complexing with air nanobubbles through electrostatic and hydrophobic interactions. The nanoparticles at pH 6 slightly shrunk during the 30-day refrigeration at 4 °C compared to those at pH 4, and the original nanostructures could revive after replenishing with fresh air nanobubble suspensions, indicating their high stability for future potential applications in food structure design innovation.
Nanobubble (NB) technologies have received considerable attention for various applications due to their low cost, eco-friendliness, scale-up potential, process control, and unique physical characteristics. NB stands for nanoscopic gaseous cavities, typically <1 μm in diameter. NBs can exist on surfaces (surface or interfacial NBs) and be dispersed in a bulk liquid phase (bulk NBs). Compared to the microbubbles, NBs exhibit high specific surface area, negative surface charge, and better adsorption. Bulk NBs can be generated by hydrodynamic/acoustic cavitation, electrolysis, water-solvent mixing, nano-membrane filtration, and so on. NBs exhibit extraordinary longevity compared to microbubbles, prompting the interest of the scientific community aiming for potential applications including medicine, agriculture, food, wastewater treatment, surface cleaning, and so on. Based on the limited amount of research work available regarding the influence of NBs on food matrices, further research, however, needs to be done to provide more insights into its applications in food industries. This review provides an overview of the generation methods for NBs, techniques to evaluate them, and a discussion of their stability and several applications in various fields of science were discussed. However, recent studies have revealed that, despite the many benefits of NB technologies, several NB generating approaches are still limited in their application in specific agro-food industries. Further study should focus on process optimization, integrating various NB generation techniques/combining with other emerging technologies in order to achieve rapid technical progress and industrialization of NB-based technologies.
Nanobubbles (NBs) have attracted increasing attention due to their unique physicochemical characteristics and enormous potential industrial applications in the biology, environment, medicine, and many other fields. A complete understanding of the underlying stability mechanism of NBs is an essential requirement for their research, development, and industrial applications. In this study, a facile and reliable approach to generating different gas NBs using a porous ceramic membrane is reported. In addition, an improved method of statistical analysis to predict NB size distribution is proposed. The results indicated that the gas NBs presented a log‐normal distribution due to an underlying Ostwald ripening process. The concentration of NBs in solution was controlled by the gas type owing to different magnitude of molecular polarizability of dissolved gases. The mean size of the NB was inversely proportional to the surface charge density. Moreover, the NB stability lies in a special state at the bubble interface, not only because the presence of surface charges hinders collisions between bubbles, but also because the degree of gas supersaturation of molecules around bubbles resulting in gas exchange has a certain stabilizing effect, and also controls the evolution of particle size distribution (PSD) for different bulk NBs. A comprehensive understanding of the underlying stability mechanism of nanobubbles (NBs) and a reliable approach to generating NBs using porous ceramic membrane are reported. The results indicated the gas NBs presented a log‐normal distribution due to an underlying Ostwald ripening process through an improved method of statistical analysis, and bubble size evolution is controlled by the gas type owing to different magnitude of molecular polarizability of dissolved gases.
The study in the field of surface nanobubbles has received great progresses since surface nanobubbles were imaged with atomic force microscopy in 2000. Meanwhile, the interest in developing new flotation techniques for separating fine particles keeps increasing. Surface nanobubbles are showing a great potential of application in the field of mineral processing. The formation of agglomerations composed of fine minerals and the easier attachment of a mineral particle to a flotation bubble are the mechanism for the improved flotation performance in the presence of surface nanobubbles. In this work, various aspects regarding surface nanobubbles including the methods for generation, measurement and identification of surface nanobubbles, the responses of surface nanobubbles to external stimuli including pH, salts, surfactants, temperature, pressure, ultrasonication et al., where flotation is usually performed are summarized. At last, we reviewed recent progresses for surface nanobubbles in flotation. This review gives perspectives for further research in the future.
It has been suggested that electrostatic stress arising from charges accumulated at the surface of nanobubbles might balance Laplace pressure leading to their stability. This mechanism has been widely discussed in the nanobubble field for the past decade. However, the stress in the diffusive double layer was overlooked when calculating the electrostatic effect in previous theories. In this communication, we recalculated this effect using the classical double layer theory. Combined with experimentally measured zeta potential, we find that the ratio of electrostatic pressure to Laplace pressure is much less than 10-2, which suggests that electrostatic interaction may not be the main factor for stabilizing bulk nanobubbles.
Characterizing the segmental dynamics of proteins, and intrinsically disordered proteins in particular, is a challenge in biophysics. In this study, by combining data from broadband dielectric spectroscopy (BDS) and both depolarized (DDLS) and polarized (PDLS) dynamic light scattering, we were able to determine the dynamics of a small peptide [ε-poly(lysine)] in water solutions in two different conformations (pure β-sheet at pH = 10 and a more disordered conformation at pH = 7). We found that the segmental (α-) relaxation, as probed by DDLS, is faster in the disordered state than in the folded conformation. The water dynamics, as detected by BDS, is also faster in the disordered state. In addition, the combination of BDS and DDLS results allows us to confirm the molecular origin of water-related processes observed by BDS. Finally, we discuss the origin of two slow processes (A and B processes) detected by DDLS and PDLS in both conformations and usually observed in other types of water solutions. For fully homogeneous ε-PLL solutions at pH = 10, the A-DLS process is assigned to the diffusion of individual β-sheets. The combination of both techniques opens a route for understanding the dynamics of peptides and other biological solutions.
Electrocatalytic generation of nanometre gas bubbles (nanobubbles) and their tuning are important for many energy and chemical processes. Studies have sought to use indirect or ex situ methods to investigate the dynamics and properties of nanobubbles, which are of fundamental interest. Alternatively, we present a molecular dynamics simulation method, which features in situ and high spatial resolution, to directly address these fundamentals. Particularly, our simulations can quantitatively reproduce the generation of ultra-stable and ultra-dense nanobubbles observed in electrochemical experiments. More importantly, our results demonstrate that the classical nucleation theory is still valid even for the scale down to several nanometres, to predict the dynamics and properties of nanobubbles. This provides general guidelines to design efficient nanocatalysts and nanoelectrodes. In our specific case, nanoelectrodes with wetting angles below 71° can suppress the generation of surface nanobubbles.
There are still fundamental problems lying in the basic research of bulk nanobubbles. Are the bulk nanobubbles reported in the literature nano scale bubbles or contaminants in fact? At present, there is not yet sufficient experimental evidence to show that the bulk nanoparticles are only gas bubbles but not other nano scale contaminants. If they are indeed nanobubbles, what causes the bulk nanobubbles observed in the literature to be much more stable than being predicted by the Epstein-Plesset theory?
This paper firstly discusses the contradiction between the traditional theory prediction and the observed lifetime of the bulk nanobubbles, and then discusses whether the so-called nanobubbles are gas aggregates. We review the existing typical models, and the influence of different conditions on the stability of bulk nanobubbles, for paving the road to a clear understanding of the stability mechanism of bulk nanobubbles. In addition, the representative production methods and characterization methods of bulk nanobubbles are discussed in order to offer some guidance to their wide range of commercial applications.
Microbubbles are very fine bubbles that shrink and collapse underwater within several minutes, leading to the generation of free radicals. Electron spin resonance spectroscopy (ESR) confirmed the generation of hydroxyl radicals under strongly acidic conditions. The drastic environmental change caused by the collapse of the microbubbles may trigger radical generation via the dispersion of the elevated chemical potential that had accumulated around the gas−water interface. The present study also confirmed the generation of ESR signals from the microbubble-treated waters even after several months had elapsed following the dispersion of the microbubbles. Bulk nanobubbles were expected to be the source of the spin-adducts of hydroxyl radicals. Such microbubble stabilization and conversion might be caused by the formation of solid microbubble shells generated by iron ions in the condensed ionic cloud around the microbubble. Therefore, the addition of a strong acid might cause drastic changes in the environment and destroy the stabilized condition. This would restart the collapsing process, leading to hydroxyl radical generation.
Some recent studies have shown that surface and interface played an important role in the assembly and aggregation of amyloid proteins. However, it is unclear how the gas-liquid interface affects protein assembly at the nanometer scale although the presence of gas-liquid interfaces is very common in in vitro experiments. Nanobubbles have a large specific surface area, which provide a stage for interactions with various proteins and peptides on the nanometer scale. In this work, nanobubbles produced in solution were employed for studying the effects of gas-liquid interface on the assembly of glucagon proteins. Atomic force microscopy (AFM) studies showed that nanobubble-treated glucagon solution formed fibrils with an apparent height of 4.02 nm ± 0.71 nm, in contrast to the fibrils formed with a height of 2.14 nm ± 0.53 nm in the control. Transmission electron microscopy (TEM) results also showed that nanobubbles promoted the assembly of glucagon to form more fibrils. Thioflavin T (ThT) fluorescence and fourier transform infrared (FTIR) analysis indicated that the nanobubbles induced the change of the glucagon conformation to a β-sheet structure. A mechanism that explains how nanobubbles affect the assembly of glucagon amyloid was proposed based on the above-mentioned experimental results. Given the fact that there are a considerable amount of nanobubbles existing in protein solutions, our results indicate that nanobubbles should be considered in fully understanding protein aggregation events in vitro.
An understanding of the acoustic cavitation threshold is essential for minimizing cavitation bio-effects in diagnostic ultrasound and for controlling cavitation-mediated tissue ablation in focused ultrasound procedures. The homogeneous cavitation threshold is an intrinsic material property of recognized importance to biomedical ultrasound as well as a variety of other applications requiring cavitation control. However, measurements of the acoustic cavitation threshold in water differ from those predicted by classic nucleation theories. This persistent discrepancy is explained by combining recently developed methods for acoustically nucleating single bubbles at threshold with numerical modeling to obtain a nucleus size distribution consistent with first-principles estimates for ion-stabilized nuclei. We identify acoustic cavitation at threshold as a reproducible subtype of heterogeneous cavitation with a characteristic nucleus size distribution. Knowledge of the nucleus size distribution could inspire new approaches to achieving cavitation control in water, tissue and a variety of other media.
Understanding the entrainment behaviour of gangue minerals in the presence of nanobubbles is necessary for the application of nanobubble technology in froth flotation. In this study, the entrainment of kaolinite particles in flotation with nanobubble water produced by decompression, and tap water, was investigated. Compared to tap water, nanobubble water enhanced the entrainment of kaolinite particles in flotation. Rheology measurements together with settling tests were further employed to examine the effect of nanobubbles on clay particle association and its correlation with the entrainment of kaolinite particles. The presence of nanobubbles appears to induce and stabilise E–E contacts of kaolinite platelets, resulting in the formation of porous three-dimensional structures. These open structures with abundant interstitial voids had a low settling velocity and therefore were more readily recovered by entrainment.
Bulk nanobubbles have unique properties and have found potential applications in many important processes. However, their stability or long lifetime still needs to further understand and draw much attention from researchers. In this letter, we generated bulk nanobubbles based on ethanol-water exchange, a method which is generally used in the studies of surface nanobubbles. The formation and stability of them was further studied by using "Nanosight" (a new type of dynamic light scattering). Our results showed that the concentration of the bulk nanobubbles produced by our method was about five times than those in the degassed group, which indicated the existence of the bulk gas nanobubbles. The effects of ethanol/water ratios and temperature on the stability of the bulk nanobubbles have also been studied and found that their numbers will reach to the maximum at the ratio of about 1:10 (v/v).
Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artifact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g., of oil) on (hydrophobic) substrates immersed in water, as they show similar properties and can easily be confused with surface nanobubbles and as they are produced in a similar way, namely, by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation. The review starts with how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. Molecular dynamic simulations and theories to account for the long lifetime of the surface nanobubbles are then reported on. The crucial element contributing to the long lifetime of surface nanobubbles and nanodroplets is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace's equation, and Henry's law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas or oil oversaturation ζ that determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation. The review also covers the potential technological relevance of surface nanobubbles and nanodroplets, namely, in flotation, in (photo)catalysis and electrolysis, in nanomaterial engineering, for transport in and out of nanofluidic devices, and for plasmonic bubbles, vapor nanobubbles, and energy conversion. Also given is a discussion on surface nanobubbles and nanodroplets in a nutshell, including theoretical predictions resulting from it and future directions. Studying the nucleation, growth, and dissolution dynamics of surface nanobubbles and nanodroplets will shed new light on the problems of contact line pinning and contact angle hysteresis on the submicron scale. © 2015 American Physical Society.
Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N2 nanobubbles at Pt nanodisk electrodes (6 - 90 nm) via the irreversible electrooxidation of hydrazine (N2H4 → N2 + 4H+ + 4e-). The nucleation and growth of a stable N2 nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N2H4 to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N2 required for nanobubble nucleation, , obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ~0.11 M, and is independent of the electrode radius and the bulk N2H4 concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N2 near the electrode surface, consistent with previously reported studies of the electrogeneration of H2 nanobubbles. is ~160 times larger than the N2 saturation concentration at room temperature and atmospheric pressure. The residual current for N2H4 oxidation after formation of a stable N2 nanobubble at the electrode surface is proportional to the N2H4 concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N2 nanobubble is determined by N2H4 electrooxidation at the three phase contact line.
Micropancakes are quasi-two-dimensional micron-sized domains on crystalline substrates (e.g. highly oriented pyrolytic graphite (HOPG)) immersed in water. They are only a few nanometers thick, and are suspected to come from the accumulation of dissolved air at the solid-water interface. However, the exact chemical nature and basic physical properties of micropancakes have been under debate ever since their first observation, primarily due to the lack of a suitable characterization technique. In this study, the stiffness of micropancakes at the interface between HOPG and ethanol-water solutions was investigated by using PeakForce Quantitative NanoMechanics (PF-QNM) mode Atomic Force Microscopy (AFM). Our measurements showed that micropancakes were stiffer than nanobubbles, and for bilayer micropancakes, the bottom layer in contact with the substrate was stiffer than the top one. Interestingly, the micropancakes became smaller and softer with an increase in the ethanol concentration in the solution, and were undetectable by AFM above a critical concentration of ethanol. But they re-appeared after the ethanol concentration in the solution was reduced. Clearly the evolution and stiffness of the micropancakes were dependent on the chemical composition in the solution, which could be attributed to the correlation of the mechanical properties of the micropancakes with the surface tension of the liquid phase. Based on the "go-and-come" behaviors of micropancakes with the ethanol concentration, we found that the micropancakes could actually tolerate the ethanol concentration much higher than 5%, a value reported in the literature. The results from this work may be helpful in alluding the chemical nature of micropancakes.
Surface nanobubbles are experimentally known to survive for days at hydrophobic surfaces immersed in gas-oversaturated water. This is different from bulk nanobubbles, which are pressed out by the Laplace pressure against any gas oversaturation and dissolve in submilliseconds, as derived by Epstein and Plesset [J. Chem. Phys. 18, 1505 (1950)JCPSA60021-960610.1063/1.1747520]. Pinning of the contact line has been speculated to be the reason for the stability of the surface nanobubbles. Building on an exact result by Popov [Phys. Rev. E 71, 036313 (2005)PLEEE81539-375510.1103/PhysRevE.71.036313] on coffee stain evaporation, here we confirm this speculation by an exact calculation for single surface nanobubbles. It is based only on (i) the diffusion equation, (ii) Laplace pressure, and (iii) Henry's equation, i.e., fluid dynamical equations which are all known to be valid down to the nanometer scale. The crucial parameter is the gas oversaturation ζ of the liquid. At the stable equilibrium, the gas overpressures due to this oversaturation and the Laplace pressure balance. The theory predicts how the contact angle of the pinned bubble depends on ζ and the surface nanobubble's footprint lateral extension L. It also predicts an upper lateral extension threshold for stable surface nanobubbles to exist.
Surface nanobubbles emerging at solid-liquid interfaces show extreme stability. In this paper, the stability of surface nanobubbles in degassed water is discussed and investigated by AFM. The result demonstrates that surface nanobubbles are kinetically stable and the liquid/gas interface is gas impermeable. The force modulation experiment further proves that there is a layer coating on nanobubbles. These critical properties suggest that surface nanobubbles may be stabilized by a layer which has a great diffusive resistance.
We present a theoretical model for the experimentally found but counterintuitive exceptionally long lifetime of surface nanobubbles. We can explain why, under normal experimental conditions, surface nanobubbles are stable for many hours or even up to days rather than the expected microseconds. The limited gas diffusion through the water in the far field, the cooperative effect of nanobubble clusters, and the pinned contact line of the nanobubbles lead to the slow dissolution rate. DOI: 10.1103/PhysRevLett.110.054501
The origin of surface nanobubbles stability is a controversial topic since nanobubbles were first observed. Here, we propose a mechanism that the three-phase contact line pinning, which results from the intrinsic nanoscale physical roughness or chemical heterogeneities of substrates, leads to stable surface nanobubbles. Using the constrained lattice density functional theory (LDFT) and kinetic LDFT, we prove thermodynamically and dynamically that the state with nanobubbles is in fact a thermodynamical metastable state. The mechanism consistent with the classical nucleation theory can interpret most of experimental characteristics for nanobubbles qualitatively, and predict relationships among the gas-side nanobubble contact angle, nanobubble size, and chemical potential.
The great implication of nanobubbles at a solid/water interface has drawn wide attention of the scientific community and industries. However, the fundamental properties of nanobubbles remain unknown as yet. In this paper, the temperature effects on the morphology of nanobubbles at the mica/water interface are explored through the combination of AFM direct image with the temperature control. The results demonstrate that the apparent height of nanobubbles in AFM images is kept almost constant with the increase of temperature, whilst the lateral size of nanobubbles changes significantly. As the temperature increases from 28 degrees C to 42 degrees C, the lateral size of nanobubbles increases, reaching a maximum at about 37 degrees C and then decreases at a higher temperature. The possible explanation for the size change of nanobubbles with temperature is suggested.
Ignition of exothermic chemical reactions in small volumes is considered as difficult or impossible due to the large surface-to-volume ratio. Here observation of the spontaneous reaction is reported between hydrogen and oxygen in bubbles whose diameter is smaller than a threshold value around 150 nm. The effect is attributed to high Laplace pressure and to fast dynamics in nanobubbles and is the first indication on combustion in the nanoscale. In this study the bubbles were produced by water electrolysis using successive generation of H(2) and O(2) above the same electrode with short voltage pulses in the microsecond range. The process was observed in a microsystem at current densities >1000 A/cm(2) and relative supersaturations >1000.
It is a long-standing hypothesis that the bubbles which evolve as a result of decompression have their origin in stable gas micronuclei lodged in hydrophobic crevices, micelles of surface-active molecules, or tribonucleation. Recent findings supported by atomic force microscopy have indicated that tiny, flat nanobubbles form spontaneously on smooth, hydrophobic surfaces submerged in water. We propose that these nanobubbles may be the gas micronuclei responsible for the bubbles that evolve to cause decompression sickness. To support our hypothesis, we used hydrophilic and monolayer-covered hydrophobic smooth silicon wafers. The experiment was conducted in three main stages. Double distilled water was degassed at the low pressure of 5.60 kPa; hydrophobic and hydrophilic silicon wafers were placed in a bowl of degassed water and left overnight at normobaric pressure. The bowl was then placed in the hyperbaric chamber for 15 h at a pressure of 1013 kPa (=90 m sea water). After decompression, bubbles were observed and photographed. The results showed that bubbles only evolved on the hydrophobic surfaces following decompression. There are numerous hydrophobic surfaces within the living body (e.g., in the large blood vessels), which may thus be the sites where nanobubbles that serve as gas micronuclei for bubble evolution following decompression are formed.
Recent experiments have convincingly demonstrated the existence of surface nanobubbles on submerged hydrophobic surfaces. However, classical theory dictates that small gaseous bubbles quickly dissolve because their large Laplace pressure causes a diffusive outflux of gas. Here we suggest that the bubbles are stabilized by a continuous influx of gas near the contact line, due to the gas attraction towards hydrophobic walls [Dammer and Lohse, Phys. Rev. Lett. 96, 206101 (2006); 10.1103/PhysRevLett.96.206101Zhang, Phys. Rev. Lett.10.1103/PhysRevLett.98.136101 98, 136101 (2007); 10.1103/PhysRevLett.98.136101Mezger, J. Chem. Phys. 128, 244705 (2008)10.1063/1.2931574]. This influx balances the outflux and allows for a metastable equilibrium, which, however, vanishes in thermodynamic equilibrium. Our theory predicts the equilibrium radius of the surface nanobubbles, as well as the threshold for surface nanobubble formation as a function of hydrophobicity and gas concentration.
Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to -6 MPa. We denote this feature as superstability.
The nucleation and stability of nanoscale gas bubbles located at a solid/liquid interface are attracting significant research interest. It is known that the physical and chemical properties of the solid surface are crucial for the formation and properties of the surface nanobubbles. Herein, we experimentally and numerically investigated the formation of nanobubbles on nanostructured substrates. Two kinds of nanopatterned surfaces, namely, nanotrenches and nanopores, were fabricated using an electron beam lithography technique and used as substrates for the formation of nanobubbles. Atomic force microscopy images showed that all nanobubbles were selectively located on the hydrophobic domains but not on the hydrophilic domains. The sizes and contact angles of the nanobubbles became smaller with a decrease in the size of the hydrophobic domains. The results indicated that the formation and stability of the nanobubbles could be controlled by regulating the sizes and periods of confinement of the hydrophobic nanopatterns. The experimental results were also supported by molecular dynamics simulations. The present study will be very helpful for understanding the effects of surface features on the nucleation and stability of nanobubbles/nanodroplets at a solid/liquid interface.
We follow the history of nanobubbles from the earliest experiments pointing to their existence to recent years. We cover the effect of Laplace pressure on the thermodynamic stability of nanobubbles and why this infers that nanobubbles are thermodynamically never stable. Therefore understanding bubble stability becomes a consideration of the rate of bubble dissolution, so the dominant approach to understanding this is discussed. Bulk nanobubbles (or fine bubbles) are treated separately from surface nanobubbles as this reflects their separate histories. For each class of nanobubbles, we look at the early evidence for their existence, methods for the production and characterization of nanobubbles, evidence that they are indeed gaseous, or otherwise, and theories for their stability. We also look at applications of both surface and bulk nanobubbles.
Froth flotation is a widely used, cost effective particle separation process. However, its high performance is limited to a narrow particle size range between approximately 50 to 600 μm for coal and 10 to 100 μm for minerals. Outside this range, the efficiency of froth flotation decreases significantly, especially for difficult-to-float particles of weak hydrophobicity (e.g., oxidized coal).
This study was aimed at enhancing recovery of an Illinois fine coal sample using a specially designed flotation column featuring a hydrodynamic cavitation nanobubble generator. Nanobubbles that are mostly smaller than 1 μm can be formed selectively on hydrophobic coal particles from dissolved air in coal slurry. Results indicate that the combustible recovery of a − 150 μm coal increased by 5–50% in the presence of nanobubbles, depending on process operating conditions. Nanobubbles also significantly improved process separation efficiency. Other major advantages of the nanobubble flotation process include lower frother dosage and air consumption since nanobubbles are produced from air naturally dissolved in water, thereby resulting in considerably lower operating costs.
The electrochemical generation of individual H2 nanobubbles at Pt nanodisk electrodes immersed in a 0.5 M H2SO4 solution is reported. A sudden drop in current associated with the transport-limited reduction of protons is observed in the i–V response at Pt nanodisk electrodes with radii of less than 50 nm. This decrease in current (95% blockage) corresponds to the formation of a single H2 nanobubble attached to the nanoelectrode that blocks proton transport to the surface. The current at which nanobubble formation occurs, inbp, is independent of scan rate and H2SO4 concentration (for [H2SO4] > 0.1 M), indicating a critical concentration profile of electrogenerated H2 required to nucleate a nanobubble. Finite element simulation based on Fick’s first law, combined with the Young–Laplace equation and Henry’s law, indicates that the concentration of H2 near the nanoelectrode surface at inbp exceeds the saturation concentration necessary to generate a nanobubble with a size comparable to the electrode size. The rapid dissolution of the nanobubble due to the high inner Laplace pressure is precisely balanced by the electrogeneration of H2 at the partially exposed Pt surface, resulting in a dynamically stabilized nanobubble. Preliminary measurements of the i–t response during nanobubble formation indicate a two-step nucleation and growth mechanism with time scales on the order of 100 μs (or less) and 1 ms, respectively.
Gas bubbles of nanometer size were produced on atomically flat solid surfaces and imaged by atomic force microscopy (AFM) in tapping mode in water. In AFM images, nanobubbles appeared like bright spheres. Some of the bubbles remained stable for hours during the experiments. The bubbles were disturbed under high load during AFM imaging. A related mechanism is discussed. © 2000 American Vacuum Society.
It is a long-standing hypothesis that the bubbles which evolve as a result of decompression have their origin in stable gas micronuclei. In a previous study (Arieli & Marmur, Respir Physiol Neurobiol, 2011), we used hydrophilic and monolayer-covered hydrophobic smooth silicon wafers to show that nanobubbles formed on a flat hydrophobic surface may be the gas micronuclei responsible for the bubbles that evolve to cause decompression sickness. On decompression, bubbles appeared only on the hydrophobic wafers. The purpose of the present study was to examine the dynamics of bubble evolution. The numbers of bubbles after decompression were greater with increasing hydrophobicity. Bubbles appeared after decompression from 150kPa, and their density increased with elevation of the exposure pressure (and supersaturation), up to 400kPa. The normal force of attraction between the hydrophobic surface and the bubble, as determined from the volume of bubbles leaving the surface of the wafer, was 38×10(-5) Newtons and the tangential force was 20×10(-5) Newtons. We discuss the correlation of these results with previous reports of experimental decompression and bubble formation, and suggest to consider appropriate modification of decompression models.
Interfacial nanobubbles (INBs) on a solid surface in contact with water (INBs) have drawn widespread research interest. Although several theoretical models have been proposed to explain their apparent long lifetimes, the underlying mechanism still remains in dispute. In this work, the morphological evolution of INBs was examined in air-equilibrated and partially-degassed water with the use of Atomic Force Microscopy (AFM). Our results show that (1) INBs shrank in the partially-degassed water while they slightly grew in the air-equilibrated water, (2) the three-phase boundary of the INBs was pinned during the morphological evolution of the INBs. Our analyses show that (1) the lifetime of INBs was sensitive to the saturation level of dissolved gases in the surrounding water, especially when the concentration of dissolved gases was close to the saturation, (2) the pinning of the three-phase boundary could significantly slow down the kinetics of both the growth and the shrinkage of the INBs. We developed one-dimensional version of the Epstein-Plesset model of gas diffusion to account for the effect of the pinning.
Nanobubbles can be observed with optical microscopy using the total-internal-reflection-fluorescence excitation. We report on total-internal-reflection-fluorescence visualization using rhodamine 6G at 5 μM concentration which results in strongly contrasting pictures. The preferential absorption and the high spatial resolution allow us to detect nanobubbles with diameters of 230 nm and above. We resolve the nucleation dynamics during the water-ethanol-water exchange: within 4 min after exchange the bubbles nucleate and form a stable population. Additionally, we demonstrate that tracer particles near to the nanobubbles are following Brownian motion: the remaining drift flow is weaker than a few micrometers per second at a distance of 400 nm from the nanobubble's center.
Microboiling events associated with the fast transient heating of a micrometer-scale metallic thin film heater immersed in water have been studied. The effect of surface properties on the microboiling transients was examined by modifying the heater surfaces with hydrophobic and hydrophilic alkanethiol self-assembled monolayers (SAMs). The microheaters are thin films of platinum or gold-plated platinum that are approximately tens of micrometers in width and hundreds of micrometers in length. The microheaters are immersed in water and rapidly heated with short (<10 μs) square voltage pulses. The temperature−time transients of the microheaters are obtained by measuring the heater resistance during the application of the heating pulse. The bubble nucleation event associated with boiling is signaled in the temperature−time transient by an inflection point that results from a change in heat transfer when a vapor bubble forms on the heater. Because of the extremely high heating rates (>108 K/s), superheating occurs and nucleation temperatures as high as 296 °C have been measured in water. The surfaces of the gold-plated heaters were coated with a series of hydrophilic [HO(CH2)6SH, HO(CH2)11SH, and HO(CH2)16SH] and hydrophobic [CH3(CH2)7SH, CH3(CH2)11SH, and CH3(CH2)15SH] SAMs. Dramatic differences are observed in the temperature−time transients of the hydrophilic versus hydrophobic SAM-coated microheaters. Microheaters modified with hydrophobic SAMs exhibit lower boiling nucleation temperatures, more pronounced inflection points, and higher average temperatures during microboiling. These differences can be rationalized by considering simple models of surface wetting and surface vapor bubble formation.
The formation of very small gas bubbles (so-called “nanobubbles”) at structured solid−water interfaces has been studied using the tapping mode atomic force microscopy (TMAFM) imaging technique. Silicon oxide wafer surfaces were prepared with different degrees of nanometer scale surface roughness and hydrophobicity. Small bubbles do not form on smooth, hydrophilic, or dehydroxylated silicon oxide wafer surfaces immersed in aqueous solutions under known levels of gas supersaturation. Randomly distributed small bubbles were observed over the whole surface of observation on methylated surfaces of controlled roughness. Bubbles formed on rough, methylated surfaces were larger and less-densely distributed than those on a smooth surface of similar hydrophobicity. The process of bubble coalescence was observed as a function of time. The macroscopic contact angle, measured with respect to the aqueous or gas phase, is very different from the microscopic contact angle detected by TMAFM and appears to be due to the influence of line tension at the pinned three-phase contact line. The latter has a value of −3 × 10-10 N and acts to stabilize the small bubbles, flattening them and thereby reducing the Laplace pressure.
Measurements of the forces in water between neutral hydrophobic surfaces prepared by covalent modification of glass are presented. The surfaces are stable under a variety of conditions including high temperature, high salt concentrations and with added ethanol. The forces between these surfaces have been studied under all of these different conditions. In water the force is attractive at very large surface separations, and discontinuities or steps are present in the force curves. It is suggested that the steps at the onset of the force are due to the bridging of submicroscopic bubbles or cavities between the surfaces and that it is their consequent growth with decreasing separation that causes the long-range attraction between hydrophobic surfaces. Electrolyte has a negligible effect on the range and strength of the measured forces, except at very high salt concentrations where the strength of the attractive forces and the adhesion between the surfaces increases slightly. The addition of ethanol reduces both the strength of the long range forces and the adhesion between the surfaces. On the basis of the comparison between these results and earlier measurements, it appears that the attraction does not obey the Derjaguin approximation. Forces were also measured in the presence of a microscopic vapor cavity created by first bringing the surfaces into contact.
As predicted by classical macroscopic theory, the lifetime for nanoscale gas bubbles is extremely short. However, stable gas
nanobubbles have been experimentally observed in recent years. In this report, we theoretically show that, if the inner density
of gas bubbles is sufficiently high, the lifetime of nanobubbles can increase by at least 4 orders of magnitude, and even
approaches the timescale for experimental observations.
In recent years, the possibility of nanobubbles at the solid-liquid interface has drawn wide attention in the scientific community and industry. Thus the search for evidences for the existence of nanobubbles became a scientific hotspot. To produce interfacial nanobubbles, a systematic experiment, called the temperature difference method, is carried out by replacing low temperature water (LTW) with high temperature water (HTW) at the highly-oriented pyrolytic graphite (HOPG)-water interface. When LTW (4 °C) is mixed with HTW (25-40 °C), nanobubbles are observed by atomic force microscopy (AFM), and their size, density and total volume per square micrometer are measured. Furthermore, pancake-like gas layers and the coexistence of nanobubbles on top of the pancake layers are also observed.
The temperature dependence of nanobubbles was investigated experimentally using atomic force microscopy. By scanning the same area of the surface at temperatures from 51 °C to 25 °C it was possible to track geometrical changes of individual nanobubbles as the temperature was decreased. Interestingly, nanobubbles of the same size react differently to this temperature change; some grow whilst others shrink. This effect cannot be attributed to Ostwald ripening, since the growth and shrinkage of nanobubbles appears to occur in distinct patches on the substrate. The total nanobubble volume per unit area shows a maximum around 33 °C, which is comparable with literature where experiments were carried out with increasing temperature. This underlines the stability of surface nanobubbles.
The silicon wafer hydrophobized with OTS was immersed into water to observe the surface in-situ by tapping-mode AFM. A large number of nano-size domain images:were found on the surface. Their shapes were characterized by the height image procedure of AFM, and the differences of the properties compared to those of the bare surface were analyzed using the phase image procedure and the interaction force curves. All the results consistently implied that the domains represent the nanoscopic bubbles attached on the surface. This was confirmed by the fact that no domain was observed in the case of the surfaces hydrophobized in the AFM fluid cell without exposure to air. The apparent contact angle of the bubbles was much smaller than that expected macroscopically, which was postulated to be the reason bubbles were able to sit stably on the surface.
Small bubbles of gas are known to exist at the interface between hydrophobic solids and water. Two features of these bubbles are unexplained: the very low contact angle and the stability. A self-consistent explanation of both of these effects is that there is a film of contaminant at the air-water interface that decreases the surface tension and thus the contact angle, and also hinders diffusion of gases from the bubble, thereby increasing the lifetime. If, during the lifetime of the bubble, the surface tension increases faster than the area of the air-water decreases, the interfacial energy can lead to a stabilization of the bubbles.
Electrolysis of water is employed to produce surface nanobubbles on highly orientated pyrolytic graphite (HOPG) surfaces. Hydrogen (oxygen) nanobubbles are formed when the HOPG surface acts as a negative (positive) electrode. The coverage and volume of the nanobubbles increase with increasing voltage. The yield of hydrogen nanobubbles is much larger than the yield of oxygen nanobubbles. The growth of the individual nanobubbles during the electrolysis process is recorded in time with the help of AFM measurements and correlated with the total current. Both the size of the individual nanobubbles and the total current saturate typically after 1 min; then the nanobubbles are in a dynamic equilibrium, meaning that they do not further grow, in spite of ongoing gas production and nonzero current. The surface area of nanobubbles shows a good correlation with the nanobubble volume growth rate, suggesting that either the electrolytic gas emerges directly at the nanobubbles' surface or it emerges at the electrode's surface and then diffuses through the nanobubbles' surface. Moreover, the experiments reveal that the time constants of the current and the aspect ratio of nanobubbles are the same under all conditions. Replacement of pure water by water containing a small amount of sodium chloride (0.01 M) allows for larger currents, but qualitatively gives the same results.
Here we demonstrate that nanobubbles can be used as cleaning agents both for the prevention of surface fouling and for defouling surfaces. In particular nanobubbles can be used to remove proteins that are already adsorbed to a surface, as well as for the prevention of nonspecific adsorption of proteins. Nanobubbles were produced on highly oriented pyrolytic graphite (HOPG) surfaces electrochemically and observed by atomic force microscopy (AFM). Nanobubbles produced by electrochemical treatment for 20 s before exposure to bovine serum albumin (BSA) were found to decrease protein coverage by 26-34%. Further, pre-adsorbed protein on a HOPG surface was also removed by formation of electrochemically produced nanobubbles. In AFM images, the coverage of BSA was found to decrease from 100% to 82% after 50 s of electrochemical treatment. The defouling effect of nanobubbles was also investigated using radioactively labeled BSA. The amount of BSA remaining on a stainless steel surface decreased by approximately 20% following 3 min of electrochemical treatment and further cycles of treatment effectively removed more BSA from the surface. In situ observations indicate that the air-water interface of the nanobubble is responsible for the defouling action of nanobubbles.
Viscous flow is familiar and useful, yet the underlying physics is surprisingly subtle and complex. Recent experiments and simulations show that the textbook assumption of 'no slip at the boundary' can fail greatly when walls are sufficiently smooth. The reasons for this seem to involve materials chemistry interactions that can be controlled--especially wettability and the presence of trace impurities, even of dissolved gases. To discover what boundary condition is appropriate for solving continuum equations requires investigation of microscopic particulars. Here, we draw attention to unresolved topics of investigation and to the potential to capitalize on 'slip at the wall' for purposes of materials engineering.
It is now widely accepted that nanometer sized bubbles, attached at a hydrophobic silica surface, can cause rupture of aqueous wetting films due to the so-called nucleation mechanism. But the knowledge of the existence of such nanobubbles does not give an answer to how the subprocesses of this rupture mechanism operate. The aim of this paper is to describe the steps of the rupture process in detail: (1) During drainage of the wetting film, the apex of the largest nanobubble comes to a distance from the wetting film surface, where surface forces are acting. (2) An aqueous "foam film" in nanoscale size is formed between the bubble and the wetting film surface; in this foam film different Derjaguin-Landau-Verwey-Overbeek (DLVO) forces are acting than in the surrounding wetting film. In the investigated system, hydrophobized silica/water/air, all DLVO forces in the wetting film are repulsive, whereas in the foam film the van der Waals force becomes attractive. (3) The surface forces over and around the apex of the nanobubble lead to a deformation of the film surfaces, which causes an additional capillary pressure in the foam film. An analysis of the pressure balance in the system shows that this additional capillary pressure can destabilize the foam film and leads to rupture of the foam film. (4) If the newly formed hole in the wetting film has a sufficient diameter, the whole wetting film is destabilized and the solid becomes dewetted. Experimental data of rupture thickness and lifetime of wetting films of pure electrolyte and surfactant solutions show that the stabilization of the foam film by surfactants has a crucial effect on the stability of the wetting film.
The effects of temperature and degassing on the formation of nanobubbles at the Mica/water interface were investigated. The nanobubles were analyzed using atomic force microscopy. Nanobubles were formed by the liquids of different temperature in order to study the temperature effects. It was observed that the average density of the nanobubles was decreased when the ethanol and water were degassed. It was also observed that the density of the nanobubles were increased with the liquid temperature and showed a growth when temperature was more than 30°C.
The stability ofsurfactantless dispersions of surface chemically pure alkanes was studied in the presence and absence of dissolved gas. It was found that simply freezing and thawing a sample of oil and water results in a dispersion. A mechanism based on fingering of the insoluble oil into the aqueous phase, due to local surface tension gradients, followed by separation and nucleation into droplets, is proposed to account for this observation.
In recent years there has been an accumulation of evidence for the existence of nanobubbles on hydrophobic surfaces in water, despite predictions that such small bubbles should rapidly dissolve because of the high internal pressure associated with the interfacial curvature and the resulting increase in gas solubility. Nanobubbles are of interest among surface scientists because of their potential importance in the long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. Here we employ recently developed techniques designed to induce nanobubbles, coupled with high-resolution tapping-mode atomic force microscopy (TM-AFM) to measure some of the physical properties of nanobubbles in a reliable and repeatable manner. We have reproduced the earlier findings reported by Hu and co-workers. We have also studied the effect of a wide range of solutes on the stability and morphology of these deliberately formed nanobubbles, including monovalent and multivalent salts, cationic, anionic, and nonionic surfactants, as well as solution pH. The measured physical properties of these nanobubbles are in broad agreement with those of macroscopic bubbles, with one notable exception: the contact angle. The nanobubble contact angle (measured through the denser aqueous phase) was found to be much larger than the macroscopic contact angle on the same substrate. The larger contact angle results in a larger radius of curvature and a commensurate decrease in the Laplace pressure. These findings provide further evidence that nanobubbles can be formed in water under some conditions. Once formed, these nanobubbles remain on hydrophobic surfaces for hours, and this apparent stability still remains a well-recognized mystery. The implications for sample preparation in surface science and in surface chemistry are discussed.
It is demonstrated that de-gassed water is more effective at dispersing hydrophobic "dirt", such as liquid hydrocarbons or oils. This effect appears to be due to the reduction of natural cavitation, which would otherwise oppose the dispersion of hydrophobic liquid droplets into water. De-gassing of the oil enhances this effect still further, and this has led to a proposal for a novel cleaning process, based on using a combination of a de-gassed (hydrophobic) solvent followed by rinsing in de-gassed water. This method might be useful as an effective, detergent-free cleaning process. Also reported are some initial studies which suggest that the effect of "inert" dissolved gases on the electrical conductivity of water may need to be reconsidered.
Electrogenerated microscale bubbles that are confined at the electrode surface have already been extensively studied because of their significant influence on electrochemistry. In contrast, as far as we know, whether nanoscale bubbles exist on the electrode surface has not been experimentally confirmed yet. Here, we report the observation of electrochemically controlled formation and growth of hydrogen nanobubbles on bare highly oriented pyrolytic graphite (HOPG) surface via in-situ tapping mode atomic force microscopy (TMAFM). By using TMAFM imaging, we observed that electrochemically generated hydrogen gas led to the formation of nanobubbles at the HOPG surface. We then employed a combination of techniques, including phase imaging, ex-situ degassing, and tip perturbation, to confirm the gas origin of such observed nanobubbles. We further demonstrated that the formation and growth of nanobubbles could be well controlled by tuning either the applied voltage or the reaction time. Remarkably, we could also monitor the evolution process of nanobubbles, that is, formation, growth, coalescence, as well as the eventual release of merged microbubbles from the HOPG surface.
We show that a very thin (5-80 nm) gas phase can exist for a long time (>1 h) at the interface between a hydrophobic solid and water. We create the gas phase from CO2, which allows us to determine the chemical identity, phase state, and density via infrared spectroscopy. The average density reveals that the gas is at approximately atmospheric pressure, which explains the unexpectedly long lifetime of the gas phase under ambient conditions. The nanoscale gas phase is reproducibly created under conditions where gas solubility is varied.
A very thin layer (5-80 nm) of gas phase, consisting of discrete bubbles with only about 40 000 molecules, is quite stable at the interface between a hydrophobic solid and water. We prepare this gas phase from either ambient air or from CO(2)(g) through a solvent exchange method reported previously. In this work, we examine the interface using attenuated total internal reflection infrared spectroscopy. The presence of rotational fine structure in the spectrum of CO(2) and D(2)O proves that molecules are present in the gas phase at the interface. The air bubbles are stable for more than 4 days, whereas the CO(2) bubbles are only stable for 1-2 h. We determine the average gas pressure inside the CO(2) bubbles from the IR spectrum in two ways: from the width of the rotational fine structure (P(gas) < 2 atm) and from the intensity in the IR spectrum (P(gas) = 1.1 +/- 0.4 atm). The small difference in gas pressure between the bubbles and the ambient (1 atm) is consistent with the long lifetime. The dimensions and curvature of a set of individual bubbles was determined by atomic force microscopy. The pressures of individual bubbles calculated from the measured curvature using the Laplace equation fall into the range P(gas) = 1.0-1.7 atm, which is concordant with the average pressure measured from the IR spectrum. We believe that the difference in stability of the CO(2) bubbles and the air bubbles is due to a combination of the much lower pressure of CO(2) in the atmosphere and the greater solubility of CO(2) in water, compared to N(2) and O(2). As expected, smaller bubbles have a shorter average lifetime than larger bubbles, and the average pressure and the curvature of individual bubbles decreases with time. Surface plasmon resonance measurements provide supporting evidence that the film is in the gas state: the thin film has a lower refractive index than water, and there are few common contaminants that satisfy this condition. Interfacial gas bubbles are not ubiquitous on hydrophobic solids: bubble-free and bubble-decorated hydrophobic interfaces can be routinely prepared.
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