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Fabrication steps for nanoporous gold. (a) Photograph of spongy structure collected on a glass substrate using only NaOH. (b) Leidenfrost levitated pool of suspended nanoporous brown gold on hot plate. (c) The same solution as (b) collected inside a glass container. (a–c) Scale bar, 1 cm. Synthesis steps of solid nanoporous gold sphere from initialization (d) to final product as a sponge (e-h). Scale bar, 1 mm. (i) Scanning electron microscopy image of the spongy brown gold. Scale bar, 100 μm. (j) Higher magnification image of the same sample as in i. Scale bar, 300 nm. (k) Leidenfrost levitated pool of suspended nanoporous black gold on hot plate. (l) The same solution as (k) collected inside a glass container. (k,l) Scale bar, 1 cm.
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Many challenges face researchers developing bottom-up alternatives to traditional top-down fabrication methods. Beneath the surface there has been a growing interest in minimizing the negative environmental repercussions of the rapidly advancing field. Here, as a novel concept of green nanochemistry, we present our experimental findings on the use...
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... our results prove that water under Leidenfrost conditions participates in and/or catalyses the chemical reaction, which is also supported analytically (Supplementary Note 1 and Supplementary Fig. S4). Once the first clusters are formed, an autocatalytic pathway could set in where further ions are adsorbed and reduced on the surface of the metal clusters 33 . ...
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... Tetrachloroauric (III) acid solution (pHB6.5) and gently placing a 5-ml drop of the resulting solution on a preheated hot plate (with a constant temperature of 270 °C), a brownish red droplet is formed, containing a dense structure. The structure was collected on a glass plate where a macroscopic spongy system can be seen even by the naked eye (Fig. ...
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... as a co-reducing agent beside the sodium hydroxide to further enhance the yield. On the basis of the mentioned approach, we demonstrate the first wet chemical synthesis of a suspended nanoporous gold using levitated Leidenfrost pool (1 ml HAuCl 4 , 20mMþ 700 ml sod. citrate 1% þ 50 ml NaOH 0.5 M, pHB7.3) in a simple, cost-effective and green way (Fig. 4b). At this stage, a portion of the brown suspension was collected to glass bottles and stored for further use (Fig. 4c). Various analytical investigations (ultraviolet-visible spectroscopy for the liquid, as well as SEM and X-ray diffraction (XRD) on dried and rinsed samples) confirmed the formation of suspended plasmonic nanoporous gold ...
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... we demonstrate the first wet chemical synthesis of a suspended nanoporous gold using levitated Leidenfrost pool (1 ml HAuCl 4 , 20mMþ 700 ml sod. citrate 1% þ 50 ml NaOH 0.5 M, pHB7.3) in a simple, cost-effective and green way (Fig. 4b). At this stage, a portion of the brown suspension was collected to glass bottles and stored for further use (Fig. 4c). Various analytical investigations (ultraviolet-visible spectroscopy for the liquid, as well as SEM and X-ray diffraction (XRD) on dried and rinsed samples) confirmed the formation of suspended plasmonic nanoporous gold (Supplementary Fig. S8). On the other hand, an orange suspension of a macroscopic 3D network of fused gold particles ...
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... is expected that the gold chains are further fused and welded together to form a 3D nanoporous hard sphere. With that in mind, we further demonstrate an 'all-in-situ' fabrication of 3D nanoporous hard sphere in the Leidenfrost drop. A naked eye observation of a levitating drop of the brown gold that converted into 3D nanoporous powder is shown in Fig. 4d-h. It is worth mentioning that the nanoparticle synthesis, clustering, assembly and fusion to form an extended 3D network occurred in the levitated drop. Washing, rinsing the sample and removing of residual salt and/or byproduct, or even enlarging the size of the metal spongy can occur 'all-in-situ'' in the levitated state (Supplementary ...
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... assembly and fusion to form an extended 3D network occurred in the levitated drop. Washing, rinsing the sample and removing of residual salt and/or byproduct, or even enlarging the size of the metal spongy can occur 'all-in-situ'' in the levitated state (Supplementary Movie 6). SEM analysis proved the 3D macro- porous structure of the gold sphere (Fig. 4i,j and Supplementary Fig. S10), whereas the XRD pattern indicated the polycrystallinity of the sample (not ...
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... visualize the role of NaOH mentioned above and to further explore the potential of the mentioned method, we demonstrate the first black gold suspended in a levitated pool (Fig. 4k) and collected in a glass bottle (Fig. 4l). The black gold suspension, which we believe to cause a paradigm shift in the field of plasmonics, has been fabricated by increasing the amount of hydroxide ions, that is, NaOH to 150 ml, while keeping all the other parameters required for the fabrication of the brown gold suspension constant ...
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... visualize the role of NaOH mentioned above and to further explore the potential of the mentioned method, we demonstrate the first black gold suspended in a levitated pool (Fig. 4k) and collected in a glass bottle (Fig. 4l). The black gold suspension, which we believe to cause a paradigm shift in the field of plasmonics, has been fabricated by increasing the amount of hydroxide ions, that is, NaOH to 150 ml, while keeping all the other parameters required for the fabrication of the brown gold suspension constant (1 ml HAuCl 4 , 20 mM þ 700 ml Sod. citrate ...
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... Research on the Leidenfrost effect dates back to Johann Gottlob Leidenfrost's observation of water droplets' blistering motions on a hot surface in the eighteenth century 1,2 . Since then, intensive research has addressed this intriguing phenomenon due to its critical importance in various applications such as boiling heat transfer 3 , spray cooling 4 , electrospray printing 5,6 and additive manufacturing 7 . It is widely accepted that the continuous vapour cushion 8 formed beneath the Leidenfrost droplet eliminates the physical contact between the droplet and the surface 9 and consequently minimizes the interfacial hydrodynamic resistance 10 associated with the contact-line pinning and solid-liquid friction 11 , which is particularly useful for agile droplet manipulations 12,13 and sustained liquid transport 10,14 . ...
... This is a strong indication of the fact that increasing T w does not markedly impact the TBL propagation, whereas the structure of the TBL could be significantly altered by the microstructures. Such an effect can be examined by revisiting equation (5) to estimate the TBL propagation velocity v TBL , as shown in Fig. 5b. Increasing T w from 130 °C to 170 °C only increases v TBL by 78.6% from 6.4 × 10 −4 m s −1 to 1.1 × 10 −3 m s −1 , which are still orders of magnitude smaller than the inertia-controlled bubble expansion (U i,e ≈ 4 m s −1 ). ...
The Leidenfrost effect—the levitation and hovering of liquid droplets on hot solid surfaces—generally requires a sufficiently high substrate temperature to activate liquid vaporization. Here we report the modulation of Leidenfrost-like jumping of sessile water microdroplets on micropillared surfaces at a relatively low temperature. Compared to traditional Leidenfrost effect occurring above 230 °C, the fin-array-like micropillars enable water microdroplets to levitate and jump off the surface within milliseconds at a temperature of 130 °C by triggering the inertia-controlled growth of individual vapour bubbles at the droplet base. We demonstrate that droplet jumping, resulting from momentum interactions between the expanding vapour bubble and the droplet, can be modulated by tailoring of the thermal boundary layer thickness through pillar height. This enables regulation of the bubble expansion between the inertia-controlled mode and the heat-transfer-limited mode. The two bubble-growth modes give rise to distinct droplet jumping behaviours characterized by constant velocity and constant energy regimes, respectively. This heating strategy allows the straightforward purging of wetting liquid droplets on rough or structured surfaces in a controlled manner, with potential applications including the rapid removal of fouling media, even when located in surface cavities.
... The so-called hydrothermal technique, which uses water as the reaction medium, is the most popular green approach. Another interesting development involves the use of the Leidenfrost effect by Abdelaziz et al. to produce nanoparticles and coatings on complex objects by overheating and charging a green chemical reactor [60]. As sources of heat for the fusion of nanoparticles, microwave energy [59] and concentrated sunlight [61] have also been investigated. ...
... Of specific interest are the many alternatives to orthodox heating [62]. In another exciting development, Abdelaziz et al. recently used the Leidenfrost effect to generate an enflamed and charged green biochemical receptacle to engineer nanoparticles and glazes on composite items [60]. ...
Due to its numerous applications in industries including medical, environmental cleanup, information technology, and energy conversion, nanotechnology is one of the extreme important scientific focuses these days. Attributable to its significance, it is now necessary to promote the creation of ecologically safe and secure nanomaterials by applying the concepts of green chemistry into both their synthesis and use. The term "green" is frequently used to describe bio-based or nanotechnological processes without considering their overall ecological impact because this qualitative framework of thought does not provide minimum conditions for their use. In this situation, environmental sustainability measures can be used to assess, improve, and quantify the environmental sustainability of synthesis processes with reference to nanotechnology. Basically, Green nanotechnology depends on the ideas of green chemistry to build, produce, use, and dispose of nanoparticles in a sustainable manner.
Keywords: Green nanotechnology; safe-by-design; sustainability; green chemistry; nanomaterials; nanoecotoxicology; nanoparticles; environmental risk assessment
... The Leidenfrost method synthesized nanoparticles through the salt solution and a heater. The technique involves forming solution droplets on a hot plate at a temperature above 200 °C (depending on the type of liquid and surface), known as the Leidenfrost temperature [26][27][28][29]. The metal oxide nanoparticle formation is illustrated as follows: a drop of water is sprinkled on a hot surface at a surface temperature above the boiling point of the water. ...
... The metal oxide nanoparticle formation is illustrated as follows: a drop of water is sprinkled on a hot surface at a surface temperature above the boiling point of the water. The water drop passes through three stages as follows (1) the outer coating of the drop is evaporated as a result of touching the hot surface; (2) due to the evaporation of the outer layer of the drop, the remaining parts of the drop levitated above the hot surface which separates with a zone of the vapour; and (3) the expanded of the water drop on the hot surface leading to the fast evaporation of all droplet layers till drying leaving powder salt on the surface [28]. Understanding how metal oxide nanoparticles are formed necessitates understanding how water molecules are converted to H + and OH − ions via two different mechanisms. ...
... The water molecules are ionized due to (1) increasing precursor concentration, which aids in distorting water molecules' hydrogen bonds, thereby making it easier to ionize the water molecules [30]; and (2) heating the water to temperatures above its boiling point [31,32]. In the Leidenfrost droplet, the second step (the levitated droplet), the water molecule ionized where inside the droplet, a negative charge was observed due to the predominant hydroxide ions [28]. However, the vapor has a positive charge outside the droplet due to the formation of hydronium ions. ...
Environmental pollution is a critical issue due to its impact on humans and other organisms. An important demand nowadays is the need for a green method to synthesize nanoparticles to remove pollutants. Therefore, this study focuses for the first time on synthesizing the MoO3 and WO3 nanorods using the green and self-assembled Leidenfrost method. The XRD, SEM, BET and FTIR analyses were used to characterize the yield powder. The XRD results emphasize the formation of WO3 and MoO3 in nanoscale with crystallite sizes 46.28 and 53.05 nm and surface area 2.67 and 24.72 m² g⁻¹, respectively. A comparative study uses synthetic nanorods as adsorbents to adsorb methylene blue (MB) in aqueous solutions. A batch adsorption experiment was performed to investigate the effects of adsorbent doses, shaking time, solution pH and dye concentration to remove MB dye. The results demonstrate that the optimal removal was achieved at pH 2 and 10 with 99% for WO3 and MoO3, respectively. The experimental isothermal data follow Langmuir for both adsorbents with a maximum adsorption capacity of 102.37 and 151.41 mg g⁻¹ for WO3 and MoO3.
... This is so-called Leidenfrost phenomenon, [1][2][3][4] initially reported by Johann Gottlob Leidenfrost in the year of 1756. 1 The high mobility and vapor layer enable a Leidenfrost drop to have promising applications, such as transportation of droplets, 2,3,5-7 chemical reactors, 8,9 nanofabrication, 10 and reduction of drag force around a moving object. 11,12 On the other hand, large Leidenfrost drops are unstable, 13,14 which may affect their applications. ...
Large Leidenfrost drops may be unstable when their diameters exceed a critical value. Via theoretical and experimental investigations, this study explored the feasibility of suppressing Leidenfrost instability in a large container, by meshing the container or its central portion into rectangular elements. Thin rods were used to construct these rectangular elements. Thin rods were used to construct these rectangular elements. Leidenfrost instability was considered in four rectangular configurations. They were also the rectangular mesh elements that might be used. There were two findings. First, the diameter of the largest inscribed cylinder in a rectangular configuration was the critical dimension to determine Leidenfrost instability. Second, the threshold value of this diameter in a rectangular configuration with rod(s) was 8.9 ± 0.7 , where was the capillary length of water. It was larger than its counterparts in both a rectangular container (without the presence of a rod) and a circular container (with or without the presence of a rod), due to the strong effect of the rod in a rectangular configuration. Based on these two findings, a large rectangular container was meshed into rectangular elements using thin rods, with the diameter of the largest inscribed cylinder in each element below the threshold value. This mesh method suppressed the Leidenfrost instability in the large container.
... Research on the Leidenfrost effect dates back to Johann Gottlob Leidenfrost's observation of water droplets' blistering motions on a hot surface in the eighteenth century 1,2 . Since then, intensive research has addressed this intriguing phenomenon due to its critical importance in various applications such as boiling heat transfer 3 , spray cooling 4 , electrospray printing 5,6 and additive manufacturing 7 . It is widely accepted that the continuous vapour cushion 8 formed beneath the Leidenfrost droplet eliminates the physical contact between the droplet and the surface 9 and consequently minimizes the interfacial hydrodynamic resistance 10 associated with the contact-line pinning and solid-liquid friction 11 , which is particularly useful for agile droplet manipulations 12,13 and sustained liquid transport 10,14 . ...
... This is a strong indication of the fact that increasing T w does not markedly impact the TBL propagation, whereas the structure of the TBL could be significantly altered by the microstructures. Such an effect can be examined by revisiting equation (5) to estimate the TBL propagation velocity v TBL , as shown in Fig. 5b. Increasing T w from 130 °C to 170 °C only increases v TBL by 78.6% from 6.4 × 10 −4 m s −1 to 1.1 × 10 −3 m s −1 , which are still orders of magnitude smaller than the inertia-controlled bubble expansion (U i,e ≈ 4 m s −1 ). ...
Rapid removal of sessile liquid droplets from a substrate has thrilling applications in surface self-cleaning and anti-frosting/icing/corrosion. Yet, our understanding of interfacial phenomena including droplet wetting and phase changes on engineered surfaces remains elusive, impeding dexterous designs for agile droplet purging. Here we introduce a simple but effective method to modulate droplet jumping behaviors on micro-pillared substrates at moderate superheat of 20- 30 {\deg}C by controlling vapor bubble growth thereon. For droplets in the Wenzel state, the micropillar matrix functions as fin array for heat transfer enhancement. Therefore, by tuning the feature sizes of micropillars, one can adjust the vapor bubble growth at the droplet base from the heat-transfercontrolled mode to the inertia-controlled mode. As opposed to the relatively slow vibration jumping in seconds on short-micropillared surface, the vapor bubble growth in the inertiacontrolled mode on tall-micropillared surface leads to droplet out-of-plane jumping in milliseconds. Such rapid droplet detachment stems from vapor bubble explosion, during which the bubble expanding velocity can reach as fast as ~4 m/s. This study unveils the mechanisms of versatile jumping behaviors of droplet from a hot micro-structured surface and sheds lights on designing engineered surfaces mitigating potential damage of vapor explosion or alleviating condensate flooding.
... [1][2][3][4] The high mobility and levitated vapor layer enable a Leidenfrost drop to have promising applications, such as directional transportation of droplets, 2,3,5-7 chemical reactors, 8,9 and nanofabrication. 10 In addition, it was reported that when a Leidenfrost vapor layer formed on the surface of a heated solid sphere, it streamed around the sphere and reduced the drag force, enabling the sphere to move faster in a liquid. 11,12 On the other hand, large ...
Leidenfrost drops have demonstrated promising applications in, for example, drag reduction. However, large Leidenfrost drops may be unstable when their diameters exceed a critical value, leading to less control of such drops in their applications. In this work, through theoretical and experimental investigations, we explore the instability of a Leidenfrost drop in a circular configuration, as well as the suppression of this instability using a small rod. There are four findings. First, the diameter of the largest inscribed cylinder inside a rod-container configuration is the critical dimension to determine Leidenfrost instability. Second, in the cases of water and isopropyl alcohol, the threshold value of this diameter is 8.3 ± 0.3 , where is the capillary length of a liquid. Third, due to the specific interface profile between the liquid drop and the surrounding vapor layer, the threshold diameter of a circular container for the instability to occur is slightly larger than its counterpart in the corresponding Rayleigh-Taylor instability problem. Fourth and finally, placing a rod inside a circular container reduces the size of the largest inscribed cylinder in the container. If the diameter of this inscribed cylinder is below the threshold value, the instability inside the container is suppressed.
... Despite the numerous studies and reports, direct conclusive classification and formulation for many commonly seen electrifying phenomena of water are still missing, for example, why the liquid droplet obtains charges during electrospray. Technically, driven by a basic understanding of the interaction between water and electricity, many applications, ranging from chemistry rection, [3][4][5] energy harvesting, [6][7][8][9][10][11] and others belonged to diverse research disciplines, have emerged. ...
... For example, water on hot substrates with a temperature higher than the Leidenfrost point can acquire negative charges due to evaporation-induced charge separation. 3 Another example is balloelectrification of water, in which water broken into a swarm of droplets via sonic spray, vibrating, or lasercaused explosive boiling obtains both negative and positive charges, and water droplets with positive charges are much more prevalent than those with negative charges. [40][41][42][43] Like liquid water, solid water, that is, ice, could also obtain electrostatic charges, which mainly relies on the temperature difference-induced ion-transfer velocity difference. ...
... As mentioned, the negative charges are spontaneously generated in aqueous solutions at Leidenfrost states, which could serve as reducing agents to synthesize nanoparticles. 3Figure 7ashows the gradual formation of gold nanoparticles in an aqueous tetrachloroauric acid droplet at the Leidenfrost state, which is reflected by the color change of the droplet.3 Similarly, charges generated from underwater Leidenfrost are also harnessed to yield a size-tailored zinc peroxide nanostructure.170 ...
As the most common but indispensable matter to humankind, water usually stays in a macroscopically electric neutral state. Due to its inherent molecular polarity, however, water can be easily electrified, which builds a connection between water and electricity. Such a coupling of water and electricity abounds deep scientific basics and technological applications. The past several decades have witnessed extensive progress in studying the mutual effects between electricity and water, but a comprehensive review of its fundamentals and applications is still largely missing. In this review, we first reassess and classify the basic electrifying methods of water according to their mechanisms, then highlight how to leverage the bond nexus of water and electricity to achieve promising technical applications. We envision that this review will inspire multidisciplinary scientific communities to think and innovate more on the research of water, electricity, and their marriage. Water is an indispensable resource for humans and other lives, and electricity has been a workhorse in the development of modern society since the last century. The inherent molecular polarity of water dictates its strong electrifying ability, therefore building its connection with electricity. The coupling of water and electricity abounds deep scientific basics and technological applications. This review classifies the basic electrifying methods of water according to underlying mechanisms, and then highlights the promising technical applications based on the interplay of electricity and water.
... 9−11 The inverse Leidenfrost effect is a phenomenon in which the droplets (i.e., aqueous solution) are coated by a boiling-film and become spherical when they approach the substrate under a temperature difference that significantly exceeds the boiling point (i.e., −196°C for liquified N 2 ). 11 Although this phenomenon has shown good application potential in many fields, such as cryogenics, 9 aerospace, 12 green chemistry, 13 and surface science, 14 its main mechanism remains largely unexplored. This is primarily attributed to the boiling-film's presence on the droplet surface, which apart from restricting their contact with the reaction substrate, also inhibits their dynamic behaviors, like self-propulsion, gliding, spinning, bobbing, and bouncing. ...
Here, we report a method to prepare cryogel particles with sponge-like mechanical properties, including high porosity and high elasticity. The preparation process of the cryogel particles of poly(2-hydroxythyl methacrylate) can be summarized in the two following steps: preparation of frozen droplets using the inverse Leidenfrost effect, followed by cryo-gelation by frozen polymerization. First, a polymer precursor was dropwise added into bulk liquid nitrogen (−196°C). Then, frozen droplets were created by the inverse Leidenfrost effect, which were subsequently polymerized in liquid paraffine (−15°C). After thawing and drying, the cryogel particles were obtained. The monolithic super-macroporous structure was observed by scanning electron microscopy (SEM). The mechanical properties of the cryogel particles were studied via compression−swelling tests. At maximum compression, the particles achieved 94.3% degree of deformation; remarkably, they returned to their original shape under the swelling state. The strategy proposed herein, which combines the inverse Leidenfrost effect with a cryo-polymerization technique, could be applied to prepare various polymer particles without employing surfactants.
... However, some residual of Ag atoms on the samples surfaces should be likely present and/or the porosity fraction of the formed nanoporous Au system should not be very high. In fact, as recognized in literature, very highly porous Au presents a completely black color due to its complete absorbance of the visible light [26,27] (the visible light is completely entrapped within the nanopores) with an Ag residual not higher than 2 % [27]. So, from these first observations, we can conclude that the dealloying processes made nanoporous the samples but they are not completely nanoporous (in all their volume) and residual Ag is likely present in the samples. ...
In this work, a steady-state detection method of 50 ppm of nitrogen dioxide NO 2 gas in a polyethylene (PET) bag using surface-enhanced Raman spectroscopy (SERS) is reported for the first time. SERS was performed on different shapes of gold nanostructures, gold nanostars (GNS) and nanoporous gold film (NPG). In both, the vibrational modes assigned to the adsorbents of NO 2 molecules such as NO 2 , NO, NO − 3 , N 2 O 3 , and N 2 O 5 were detected. The generation of adsorbents is due to the photo-chemical reaction driven by the hot electrons generated at the surface of gold nanostructures. Our finding is supported by a finite element simulation excluding the plasmon-induced photo-thermal process due to the long duration of excitation compared to the duration of SERS measurement. The mechanism of NO 2 photochemical reactions is explained, confirming the adsorption of NO 2 molecules on the gold surface towards their oxygen atoms.
... This process is enhanced by the presence of salts or extreme pressure and temperature [142,143]. A similar charge separation is observed at the liquid-vapor interface of a Leidenfrost drop [106]. The hydronium ions are carried away by the vapor while the drop is left with the hydroxyl ions (Figure 8 (c1)). ...
... Similarly, plasmonic Gold nanoparticles (Figure 8 (b2) and Figure 8 (b3)), Zinc-Oxide (ZnO) nanoparticles, Copper-Oxide (CuO) nanorods and Platinum nanoparticles can be synthesized using their corresponding salt solutions. Moreover, by using additional reducing agents like citric acid or sodium hydroxide solutions, nanoporous superhydrophilic metals can be fabricated [106,145]. ...
... These Leidenfrost reactors can also be used to create nanoscale coatings [106]. For example, high performance indium oxide (In2O3) and zinc-oxide (ZnO) thin-film transistors were prepared as an alternative to traditional spray pyrolysis techniques [146]. ...
The Leidenfrost effect is a case of thin-film boiling where a drop of liquid levitates on a surface heated to temperatures significantly higher than the liquid’s boiling point. When the drop contacts this superheated surface, a thin film of vapor (typically around 100 microns) forms instantaneously between the surface and the drop. The vapor layer supports the weight of the drop and thermally shields it from immediate evaporation. Due to the absence of direct contact between the drop and the surface, the Leidenfrost effect represents the case of a perfectly hydrophobic surface. In this chapter, we discuss the effect of surface wettability on the onset of this thin-film boiling state. We discuss passive methods, such as surface texturing, and active methods, such as using external fields to alter and control the transition to the Leidenfrost effect. The absence of a contact line provides extremely high mobility to these levitating drops and virtually eliminates friction. We discuss how this reduced friction can, in one case, reduce viscous drag on solid objects and, in another case, by introducing an asymmetry in the vapor flow, induce self-propulsion of levitating drops.
