Figure - available from: Experiments in Fluids
This content is subject to copyright. Terms and conditions apply.
Crown formation and the breakup of an emulsion droplet (Donset ~ 0.2 mm) followed by the ligament-mediated breakup. The scale bar indicates 500 µm
Source publication
We delineate and examine the distinct breakup modes of evaporating water-in-oil emulsion droplets under acoustic levitation. The emulsion droplets consist of decane/dodecane/tetradecane as oil, while the water concentration is varied from 10 to 30% (v/v). The droplets were heated under different laser irradiation intensities and were observed to ex...
Citations
... horizontal and vertical radius of the droplet, respectively (droplet configuration in the inset of Fig. 1) 48,49 . Thus, ' D o = 2 × R o ' represents the onset diameter of the droplet. ...
... The phenomenon of bubble dynamics is nicely explained from the generation of bubbles to its evolution. The use of a femtosecond laser to generate microbubbles allows for exact control and prediction of the size, position, and polydispersity of the generated bubbles, making them useful in applications such as microemulsions 38,49 , laser-induce fragmentation of polymers 1,64 , and laser induced cavitation in e-fuels 6,48,51 . Furthemore, the findings could help in understanding the incubation effect and optimizing laser parameters for medical laser and nano/micro-object manipulation applications. ...
This study focuses on the bubble dynamics and associated breakup of individual droplets of diesel and biodiesel under the influence of femtosecond laser pulses. The bubble dynamics were examined by suspending the droplets in the air through an acoustically levitated setup. The laser pulse energies ranged from 25 to 1050 µJ, and droplet diameters varied between 0.25 and 1.5 mm. High-speed shadowgraphy was employed to examine the influence of femtosecond laser intensity and multiple laser pulses on various spatial–temporal parameters. Four distinct sequences of regimes have been identified, depending on early and late times: bubble creation by individual laser pulses, coalescence, bubble rupture and expansion, and droplet fragmentation. At all laser intensities, early-time dynamics showed only bubble generation, while specifically at higher intensities, late-time dynamics revealed droplet breaking. The droplet breakup is further categorized into three mechanisms: steady sheet collapse, unstable sheet breakup, and catastrophic breakup, all following a well-known ligament and secondary breakup process. The study reveals that laser pulses with high repetition rates and moderate laser energy were the optimal choice for precise bubble control and cutting.
... Meanwhile, excessive laser power risks abrupt droplet rupture. [59][60][61] However, this parameter range falls outside our regime of interest as the droplet functionality as a carrier is compromised. ...
Droplet evaporation is a complex and fundamental topic that holds great scientific interest due to its relevance in numerous physical and biological processes. We systematically study laser-induced nanofluid droplet evaporation under varying light frequencies. Our findings indicate the existence of two spectral regimes where droplet evaporation is either enhanced or inhibited, which is in stark contrast to the constant regime observed under fixed laser power. The enhanced regime is attributed to the rapid heat transfer initiated by the formation of vapor microbubbles inside the droplet, causing an increase in the overall temperature of the droplet. Conversely, the inhibited regime is associated with reduced heat conduction inside the droplet resulting from localized cooling effects brought about by droplet evaporation. Correlations between heat transfer mechanisms and thermal responses at the droplet surface further support these observations. We also demonstrate that both convective and conductive heat transfers determine the critical light frequency to enhance droplet evaporation. Three light-driven flow patterns are additionally identified inside the droplet. These are photophobic, phototropic, and rolling flows, which are driven by the explosive bubble growth, surface tension gradients, and mass shifts in the droplet center, respectively. Understanding these properties is important for developing miniature evaporators, nanoparticle self-assembly, and various biomedicine applications requiring precise temperature and kinetic control.
... In addition, the surface tension coefficient of some liquids is greatly affected by temperature, which causes the surface tension of droplets to decrease after being heated, and the droplets cannot maintain equilibrium. [25][26][27][28][29] When the distance between the two ends of the levitator is greater than the resonance distance, the position of the droplets will be unstable, and vertical oscillation will occur. 30,31 At this point, the oscillation amplitude of the droplet may remain constant or increase exponentially, 30,32 and the quotient of the vertical translational frequency divided by the lateral translational frequency equals the ratio of the restoring force coefficients. ...
We, herein, present dynamic behaviors of droplets entering an ultrasonic standing wave field (19 800 Hz) at different angles. In experiments, droplets’ motion is recorded by using a high-speed camera, and an in-house Python program is used to obtain droplet positions and morphological characteristics as functions of time. The experimental results indicate that when the sound intensity is lower than the instability intensity and higher than the levitation intensity, the vertically falling droplet will oscillate up and down based on the equilibrium position. Although the oscillation amplitude decays from 0.52Tl to 0.01Tl (Tl = λ/2, λ is the wavelength) under the action of viscous resistance, the oscillation frequency of the droplet remains unchanged. Meanwhile, as the droplet’s position oscillates, the acoustic radiation force on the droplet also periodically fluctuates, resulting in the acoustically forced oscillation of the droplet shape. In addition, when the droplet enters the sound field with a horizontal tilt angle θ of 15°, it undergoes a V-shaped translational motion, first descending and then ascending. As the sound pressure amplitude increases, the rebound position of the droplet advances. When the sound pressure amplitude reaches the instability value (7900 Pa), the droplet undergoes right-hand and left-hand disintegration during its descent and ascent, respectively. This instability is due to the acoustic radiation pressure distribution and the droplet’s V-shaped trajectory. This work comprehensively discussed the complex motion of moving droplets in the acoustic standing wave field, which may inspire revealing the spray motion in the liquid engine with high-intensity resonance.
... When the engine is running, a complex multi-physical field environment consisting of sound, heat, and flow was formed in the combustion chamber. [34][35][36] Therefore, the study of acoustically induced droplet breakup in multi-physical fields has also attracted attention, [37][38][39][40][41] which is of great significance in revealing the mechanism of combustion instability. Remarkably, even without changing the sound source, stable levitated droplets will also break and atomize due to thermal induction, and Basu et al. [37][38][39][40] attributed this instability to the decrease in surface tension of liquid droplets and the increase in acoustic streaming velocity caused by the increase in the surrounding gas density while Wei et al. 41 attributed it to the dramatic changes in the pressure distribution on the droplet surface due to a large number of binary molecules from the evaporating process. ...
... [34][35][36] Therefore, the study of acoustically induced droplet breakup in multi-physical fields has also attracted attention, [37][38][39][40][41] which is of great significance in revealing the mechanism of combustion instability. Remarkably, even without changing the sound source, stable levitated droplets will also break and atomize due to thermal induction, and Basu et al. [37][38][39][40] attributed this instability to the decrease in surface tension of liquid droplets and the increase in acoustic streaming velocity caused by the increase in the surrounding gas density while Wei et al. 41 attributed it to the dramatic changes in the pressure distribution on the droplet surface due to a large number of binary molecules from the evaporating process. ...
This work reports an investigation of the acoustically induced accelerated deformation of drops in high-intensity acoustic standing wave fields generated by a single-axis acoustic levitator. The dynamic characteristics of droplet deformation are obtained and discussed based on high-speed visualization and in-house Python codes. Based on the actual physical characteristics, the finite element method numerical model has been developed for intercoupling the sound field and flow field, allowing for bidirectional feedback between the drop shape and the acoustic wave. The experimental results indicate that during the deformation process of droplets, their equatorial radius expands at an increasing speed without artificially increasing the sound field intensity. The simulation shows that the acoustic radiation suction acting on the equator dominates droplet deformation. Furthermore, there is a kind of positive feedback loop between the acoustic radiation pressure (pr) amplitude at the drop’s equator and the aspect ratio (AR) during the deformation period. It is confirmed that this causes the spontaneous accelerated expansion of the droplet’s equator. In addition, the functional relationship between pr at the drop’s equator and the AR has been obtained through theoretical derivation, which is consistent with the simulation results. Finally, the critical Bond number (Ba,s) of the rim instability is also obtained. This work provides deeper insights into contactless liquid manipulation and ultrasonic atomization technology applications.
... The effect of the acoustic pressure, the surface tension of the liquid, and ambient temperature on the droplet dynamics and the stability in acoustic levitation have been studied elsewhere [30,31]. When focused on the levitated droplet, the laser beam induces cavitation in pure liquid droplets, showing three atomization regions: rapid atomization, sheet formation, and coarse fragmentation [32][33][34][35]. The influence of nanosecond laser-induced shock waves and stress waves in liquids with different properties have also been studied in detail [27,[36][37][38][39]. ...
... The ligament attached to the sheet becomes thick at t = 1.3 ms, and subsequently, it becomes Rayleigh-Plateau unstable, resulting in the droplet ejection from the edge of the sheet (t = 1.3 ms). A similar type of sheet breakup has also been reported in emulsion droplets [35] and water droplets [32]. ...
... In the present work, the ligaments having an aspect ratio larger than π (~3.14) were observed to undergo pinch-off, which affirms that the ligament breakup occurs via the Rayleigh-Plateau instability. A similar observation has been reported in acoustically levitated evaporating as well as burning droplets [10,34,35]. The characteristic time scale associated with the ligament breakup (ligament breakup time) is estimated, and as expected, it is found to be longer for thicker ligaments. ...
Single droplet fragmentation of different liquids is essential for the fundamental understanding and augmenting of the atomization process involved in several industrial processes. Most importantly, there is a need to increase our understanding of the atomization of biofuels in combustion devices such as gas turbines and internal combustion engines. In this work, we describe and compare the laser-induced fragmentation of ethanol, Rapeseed Methyl Ester (RME), and their emulsions. We use a nanosecond laser pulse of various laser energies to fragment droplets. Acoustic levitation is used for non-contact manipulation of an isolated single droplet, and the fragmentation sequences are recorded using two high-speed cameras. Three breakup modes are observed: Droplet rupture and air entrapment, sheet breakup, and prompt/catastrophic fragmentation. At lower laser energy, air entrapment inside the droplet occurs. Sheet breakup and catastrophic breakup are observed for droplets of RME emulsions. The ligament-mediated atomization via Rayleigh-Plateau instability and the resulting secondary droplets are studied in detail. The breakup of RME-Ethanol emulsions results in the formation of small secondary droplets compared to pure liquid droplets.
... Meanwhile, the thin sheet develops a rim and consequently leads to the creation and growth of ligaments or threads. These ligaments become unstable and eventually experience breakup due to Rayleigh-Plateau instability [33][34][35] (τ c = ρ×d l 3 σ = 69 µs), where ρ and σ are the density and surface tension of liquid, and d l is the ligament diameter. When the laser energy is increased to 10 mJ, α is observed to increase (∼ 42 o ). ...
Droplet-droplet interactions is ubiquitous in various applications ranging from medical diagnostics to enhancing and optimizing liquid jet propulsion. We employ an experimental technique where the laser pulse interacts with a micron-sized droplet and causes optical breakdown. The interaction of a nanosecond laser pulse and an isolated spherical droplet is accurately controlled and manipulated to influence the deformation and fragmentation of an array of droplets. We elucidate how the fluid dynamic response (such as drop-drop and shock-drop interactions) of an arrangement of droplets can be regulated and optimally shaped by laser pulse energy and its interplay with the optical density of liquid target. A new butterfly type breakup is revealed, which is found to result in controlled and efficient fragmentation of the outer droplets in an array. The spatio-temporal characteristics of a laser-induced breakdown dictate how shock wave and central droplet fragments can influence outer droplets. The incident laser energy and pulse width employed in this work are representative of diverse industrial applications such as surface cleaning, nano-lithography, microelectronics, and medical procedures such as intraocular microsurgery.
... Meanwhile, the thin sheet develops a rim and consequently leads to the creation and growth of ligaments or threads. These ligaments become unstable and eventually experience breakup due to Rayleigh-Plateau instability[32][33][34]. The experimental ligament breakup time (∼ 75 µs) is consistent with the capillary timescale (τ c = ρ×d l 3 σ = 69 µs), where ρ and σ are the density and surface tension of liquid, and d l is the ligament diameter. ...
Droplet-droplet interactions is ubiquitous in various applications ranging from medical diagnostics to enhancing and optimizing liquid jet propulsion. We employ an experimental technique where the laser pulse interacts with a micron-sized droplet and causes optical breakdown. The synergy of a nanosecond laser pulse and an isolated spherical droplet is accurately controlled and manipulated to influence the deformation and fragmentation of an array of droplets. We elucidate how the fluid dynamic response (such as drop-drop and shock-drop interactions) of an arrangement of droplets can be regulated and optimally shaped by laser pulse energy and its interplay with the optical density of liquid target. A new butterfly type breakup is revealed, which is found to result in controlled and efficient fragmentation of the outer droplets in an array. The spatio-temporal characteristics of a laser-induced breakdown dictate how shock wave and central droplet fragments can influence outer droplets. The incident laser energy and pulse width employed in this work are representative of diverse industrial applications such as surface cleaning, nano-lithography, microelectronics, and medical procedures such as intraocular microsurgery.
... A similar phenomenology is encountered in the combustion of multi-component fuel droplets with high volatility differential (Avedisian & Andres 1978;Rao, Kamakar & Basu 2017), or with emulsions (Rao & Basu 2020). In both cases, the volatile phase nucleates a vapour bubble within the continuous liquid phase of the droplet, a bubble which burst and ejects a ligament when reaching the droplet surface. ...
... However, the breakup of droplets through the formation of patches and holes occurs only in droplets with 20% water concentration. In the other mixtures, however, the breakup mechanisms are qualitatively as well as quantitatively different, and therefore, they are not in the scope of present work (see Ref. [43]). The properties of the tested liquids are listed in Table 1 (see supplementary material for the calculation of surface tension and viscosity at the required temperature). ...
Atomization of emulsion droplets is ubiquitous across a variety of application domains ranging from NextGen combustors to fabrication of biomedical implants. An understanding of the atomization mechanism in emulsions can result in a paradigm shift in customized designs of efficient systems, be it in energy or biotechnology sectors. In this paper, we specifically study the breakup mechanism of an evaporating contact-free (acoustic levitation) emulsion droplet (water-oil) under external heating. Three distinct regimes are observed during the lifespan of the evaporating droplet. Initially, the droplet diameter regresses linearly with time, followed by vapor bubble nucleation due to a significant difference in the boiling temperature among the components of the emulsion. The collapse of this bubble results in a high-intensity breakup of the droplet leading to the propulsion of residual liquid in the form of a crown-like sheet. The area of the expanding crown varies linearly with the square of the time. It is hypothesized that the expansion of the liquid sheet centrifuges the larger water sub-droplets towards the edge, resulting in unique spatial segregation. Subsequently, we report the first observation of complex patches (representing water sub-droplets) and the rupture of the thin sheet adjacent to patches into holes (with hole growth rate ranging from 1.2 to 1.4 m/s) in the context of an evaporating isolated emulsion droplet. The hole formation results in the creation of ligaments which undergo breakup into secondary droplets with Sauter mean diameter (SMD) ∼ 50 µm.
... However, the breakup of droplets through the formation of patches and holes occurs only in droplets with 20% water concentration. In the other mixtures, however, the breakup mechanisms are qualitatively as well as quantitatively different, and therefore, they are not in the scope of present work (see Ref. [43]). The properties of the tested liquids are listed in Table 1 (see supplementary material for the calculation of surface tension and viscosity at the required temperature). ...
