Leidenfrost on a ratchet
TL;DR: A liquid droplet placed on a hot surface can levitate, and moreover, self-propel if the surface is textured as discussed by the authors, which means that the properties of the liquid are irrelevant.
Abstract: A liquid droplet placed on a hot surface can levitate, and moreover, self-propel if the surface is textured. Solids can similarly self-propel, which means that the properties of the liquid are irrelevant. Rather, it is the vapour beneath the drop that does the propelling.
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TL;DR: Topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling—by heat transfer—the liquid–gas phase transition at hot surfaces, and can potentially be applied to control other phase transitions.
Abstract: Textured superhydrophobic surfaces—well known for their water-repelling properties—can be used to control the boiling state of a liquid in contact with a hot surface, suppressing the unwanted nucleation of bubbles. Textured superhydrophobic surfaces are well known and suitably named for their water-repelling properties. Ivan Vakarelski et al. show here that such surfaces can be used to control a very different property — the boiling state of a liquid in contact with a hot surface. They find that the hot surface can be engineered such that the system remains in the 'Leidenfrost' regime, whereby boiling takes place only in a continuous vapour film at the hot surface, rather than going through the familiar 'nucleate boiling' bubbling phase. The complete suppression of nucleate boiling could be advantageous in industrial situations in which vapour explosions are best avoided — in nuclear power plants, for instance. Textured, water-repelling surfaces might also be used to control or prevent other phase transitions, such as ice or frost formation. In 1756, Leidenfrost1 observed that water drops skittered on a sufficiently hot skillet, owing to levitation by an evaporative vapour film. Such films are stable only when the hot surface is above a critical temperature, and are a central phenomenon in boiling2. In this so-called Leidenfrost regime, the low thermal conductivity of the vapour layer inhibits heat transfer between the hot surface and the liquid. When the temperature of the cooling surface drops below the critical temperature, the vapour film collapses and the system enters a nucleate-boiling regime, which can result in vapour explosions that are particularly detrimental in certain contexts, such as in nuclear power plants3. The presence of these vapour films can also reduce liquid–solid drag4,5,6. Here we show how vapour film collapse can be completely suppressed at textured superhydrophobic surfaces. At a smooth hydrophobic surface, the vapour film still collapses on cooling, albeit at a reduced critical temperature, and the system switches explosively to nucleate boiling. In contrast, at textured, superhydrophobic surfaces, the vapour layer gradually relaxes until the surface is completely cooled, without exhibiting a nucleate-boiling phase. This result demonstrates that topological texture on superhydrophobic materials is critical in stabilizing the vapour layer and thus in controlling—by heat transfer—the liquid–gas phase transition at hot surfaces. This concept can potentially be applied to control other phase transitions, such as ice or frost formation7,8,9, and to the design of low-drag surfaces at which the vapour phase is stabilized in the grooves of textures without heating10.
469 citations
TL;DR: In this paper, a comprehensive review of published literatures concerning the fluid mechanics and heat transfer mechanisms of liquid drop impact on a heated wall is provided, divided into four parts, each centered on one of the main heat transfer regimes: film evaporation, nucleate boiling, transition boiling, and film boiling.
357 citations
TL;DR: The facile water droplet printing on superamphiphobic surfaces is leveraged to create rewritable surface charge density gradients that stimulate droplet propulsion under ambient conditions17 and without the need for additional energy input.
Abstract: The directed, long-range and self-propelled transport of droplets on solid surfaces is crucial for many applications from water harvesting to bio-analysis1-9. Typically, preferential transport is achieved by topographic or chemical modulation of surface wetting gradients that break the asymmetric contact line and overcome the resistance force to move droplets along a particular direction10-16. Nonetheless, despite extensive progress, directional droplet transport is limited to low transport velocity or short transport distance. Here we report the high-velocity and ultralong transport of droplets elicited by surface charge density gradients printed on diverse substrates. We leverage the facile water droplet printing on superamphiphobic surfaces to create rewritable surface charge density gradients that stimulate droplet propulsion under ambient conditions17 and without the need for additional energy input. Our strategy provides a platform for programming the transport of droplets on flat, flexible and vertical surfaces that may be valuable for applications requiring a controlled movement of droplets17-19.
347 citations
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TL;DR: The GI/BSI/DFKI Protection Profile constitutes after the implementation of the identified improvements as the proposed evaluation methodology for remote electronic voting systems and can now be applied to available systems.
Abstract: The previous part discusses the GI/BSI/DFKI Protection Profile which constitutes after the implementation of the identified improvements as the proposed evaluation methodology for remote electronic voting systems. The result can now be applied to available systems. Currently, there is no system that has been evaluated against the GI/BSI/DFKI Protection Profile or even against the improved version.
332 citations
TL;DR: In this article, the authors show that the wetting symmetry of a droplet can be broken at high temperature by creating two concurrent thermal states (Leidenfrost and contact-boiling) on a topographically patterned surface, thus engendering a preferential motion of the droplet towards the region with a higher heat transfer coefficient.
Abstract: Directed motion of liquid droplets is of considerable importance in various water and thermal management technologies. Although various methods to generate such motion have been developed at low temperature, they become rather ineffective at high temperature, where the droplet transits to a Leidenfrost state. In this state, it becomes challenging to control and direct the motion of the highly mobile droplets towards specific locations on the surface without compromising the effective heat transfer. Here we report that the wetting symmetry of a droplet can be broken at high temperature by creating two concurrent thermal states (Leidenfrost and contact-boiling) on a topographically patterned surface, thus engendering a preferential motion of a droplet towards the region with a higher heat transfer coefficient. The fundamental understanding and the ability to control the droplet dynamics at high temperature have promising applications in various systems requiring high thermal efficiency, operational security and fidelity. Controlled motion of a droplet on a hot surface is hampered by the formation of an evaporation layer below the droplet (Leidenfrost effect). But a cleverly patterned surface induces a Leidenfrost–contact-boiling state, directing the droplet’s motion.
249 citations
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TL;DR: A surface having a spatial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill after an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides of the drop.
Abstract: A surface having a spatial gradient in its surface free energy was capable of causing drops of water placed on it to move uphill. This motion was the result of an imbalance in the forces due to surface tension acting on the liquid-solid contact line on the two opposite sides ("uphill" or "downhill") of the drop. The required gradient in surface free energy was generated on the surface of a polished silicon wafer by exposing it to the diffusing front of a vapor of decyltrichlorosilane, Cl(3)Si(CH(2))(9)CH(3). The resulting surface displayed a gradient of hydrophobicity (with the contact angle of water changing from 97 degrees to 25 degrees ) over a distance of 1 centimeter. When the wafer was tilted from the horizontal plane by 15 degrees , with the hydrophobic end lower than the hydrophilic, and a drop of water (1 to 2 microliters) was placed at the hydrophobic end, the drop moved toward the hydrophilic end with an average velocity of approximately 1 to 2 millimeters per second. In order for the drop to move, the hysteresis in contact angle on the surface had to be low (
1,446 citations
"Leidenfrost on a ratchet" refers background in this paper
...Combining equations (1), (4) and (5), and noting Fo≈ ρa4/τ 2, we find:...
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...Combining equations (5)–(7) provides the terminal velocity V ∼ (a/τ )(ρ/ρo)(λ/ε)(a/R) of Leidenfrost drops on ratchets....
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...Equation (5) shows that M o increases as R3/2, so that we can express the force F ∼M o U likely to drive the drop on the ratchet....
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TL;DR: The movement of liquid drops on a surface with a radial surface tension gradient is described here and has implications for passively enhancing heat transfer in heat exchangers and heat pipes.
Abstract: The movement of liquid drops on a surface with a radial surface tension gradient is described here. When saturated steam passes over a colder hydrophobic substrate, numerous water droplets nucleate and grow by coalescence with the surrounding drops. The merging droplets exhibit two-dimensional random motion somewhat like the Brownian movements of colloidal particles. When a surface tension gradient is designed into the substrate surface, the random movements of droplets are biased toward the more wettable side of the surface. Powered by the energies of coalescence and collimated by the forces of the chemical gradient, small drops (0.1 to 0.3 millimeter) display speeds that are hundreds to thousands of times faster than those of typical Marangoni flows. This effect has implications for passively enhancing heat transfer in heat exchangers and heat pipes.
914 citations
Book•
01 Jan 1956TL;DR: In this article, the authors discuss the two-phase flow and show that it is possible to predict the instability of the two phases of two phase flow and the two phase pool boiling crisis.
Abstract: 1.Introduction 2.Pool Boiling 3.Hydrodynamics of Two-Phase Flow 4.Flow Boiling 5.Flow Boiling Crisis 6.Instability of Two-Phase Flow Appendix References Index
574 citations
519 citations
TL;DR: It is reported that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchetlike) topology and proposed that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid.
Abstract: We report that liquids perform self-propelled motion when they are placed in contact with hot surfaces with asymmetric (ratchetlike) topology. The pumping effect is observed when the liquid is in the Leidenfrost regime (the film-boiling regime), for many liquids and over a wide temperature range. We propose that liquid motion is driven by a viscous force exerted by vapor flow between the solid and the liquid.
466 citations
"Leidenfrost on a ratchet" refers background in this paper
...Combining equations (3) and (4) gives the M o (R) variation:...
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...From equations (1)–(4), this drag also scales as R3/2 and is in the μN range....
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...Combining equations (1), (4) and (5), and noting Fo≈ ρa4/τ 2, we find:...
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