Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G12: Leidenfrost and Levitating Drops |
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Chair: Yoshiyuki Tagawa, Tokyo University of Agriculture and Technology Room: Georgia World Congress Center B217 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G12.00001: Impact characteristics of liquid nitrogen droplets Srinivas Vanapalli, Michiel Limbeek, Thomas Nes, Marcel ter Brake In spray cooling liquid drops impact the hot wall and depending on the wall temperature the drop either touches-down (contact boiling) or levitates (film boiling). The latter case results in a great reduction in heat transfer by the insulating vapour layer under the drop. Here, we study this phenomena for cryogenic systems, where we investigate the impact of a liquid nitrogen droplet on a sapphire plate. Since sapphire is transparent, we can perform high-speed frustrated total internal reflection to study the solid-liquid contact. From our experiments we obtained a phase diagram where the boiling behaviour is characterized by varying the initial prism temperature and drop velocity. Next, we utilized a stream of drops to cool down the prism from the film boiling regime down to the contact boiling regime, where we find good agreement between the heat transfer rate and the contact dynamics as predicted by our phase diagram. Our study gives insight on how the current literature established for non-cryogenic conditions can be applied to the present case of liquid nitrogen drops. |
Monday, November 19, 2018 10:48AM - 11:01AM |
G12.00002: Leidenfrost-Like Non-Coalescence of Liquid Helium Drops Matthew Wallace, David Mallin, Kenneth Langley, Andres A Aguirre Pablo, Sigurdur T Thoroddsen, Peter Taborek We present the results of our investigation of Leidenfrost-like non-coalescent impact behavior of normal liquid helium drops in an optical cryostat at saturated vapor pressure at temperatures of 2.2-5.2 K. We observe non-coalescence when drops come into contact with both solid substrates and with bulk liquid. In contrast to other well-known types of non-coalescence, our drops exist in an almost perfectly isothermal environment and in the absence of a non-condensible gas such as air. Non-coalescence can occur throughout the entire temperature range of liquid helium and is particularly robust near the liquid-vapor critical point, where drop bouncing is observed at higher impact velocities. We examine the heat and mass balance in the drop and analyze the role of curvature-induced evaporation which generates a sub-millikelvin temperature difference between the drop and the substrate; this small temperature gradient between the drop and the surface produces and evaporation-driven gas film supporting the drop. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G12.00003: Abstract Withdrawn
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Monday, November 19, 2018 11:14AM - 11:27AM |
G12.00004: High-speed measurements of Leidenfrost vapor layer oscillations and collapse Dana Harvey, Joshua Mendez, Justin Burton The thin vapor layer generated between a very hot surface and an evaporating liquid is subject to complex, hydrodynamic fluctuations. These fluctuations can excite spontaneous oscillations of Leidenfrost drops, and are the eventual progenitor of the collapse of the vapor layer and explosive boiling. Using a new electrical technique, we have characterized these fluctuations on sub-microsecond time scales. The vapor layer is treated as the dielectric of a capacitor where one conductor is a heated metallic surface and the other is a conducting liquid. The liquid is pumped with a high-frequency carrier signal so that the capacitive reactance of the vapor layer is low. The amplitude of the carrier wave is monitored in time. Collapse of the vapor layer is marked by a sharp increase in the real part of the complex electrical impedance. With this technique, coupled with high-speed video, we characterized numerous oscillations of Leidenfrost drops, including well-known star-shaped oscillations, as well and the eventual collapse the of vapor layer near a hot titanium finger immersed in a salt water bath. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G12.00005: Does a liquid has just one Leidenfrost temperature? Michiel A. J. van Limbeek, Olinka Ramirez Soto, Chao Sun, Detlef Lohse The remarkable phenomenon of the Leidenfrost effect, in which a drop levitates on its own vapor above a superheated plate, has drawn attention for centuries. Many researchers focus on at which particular temperature a liquid drop will become in the Leidenfrost state, however here we study how this temperature is affected by varying the ambient conditions. Firstly, we vary the ambient pressure and show that a strong dependency is observed for all six liquids studied. By rescaling the temperature we can collapse all data on a single curve. Secondly, by changing the temperature the surroundings, we can lower the Leidenfrost temperature by tens of Kelvins. We show that these drops however are in a metastable state and cannot recover to the Leidenfrost state after touching the plate by a disturbance. Our study shows that by varying the ambient conditions, we can manipulate the Leidenfrost temperature, which opens up a new paradigm on how to approach the complex phenomenon, which the Leidenfrost effect is. |
(Author Not Attending)
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G12.00006: Abstract Withdrawn
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Monday, November 19, 2018 11:53AM - 12:06PM |
G12.00007: A Dance of Ice and Fire: Compound Leidenfrost Films Daniel Cusumano, Saurabh Nath, Jonathan Boreyko The Leidenfrost effect is when an evaporating liquid, or a sublimating solid such as dry ice, levitates on its own vapor. Here, we demonstrate that a disk of frozen water placed on a sufficiently hot surface exhibits a compound Leidenfrost state, where both liquid and vapor films emanate from underneath the levitating ice. As the surface superheat temperature was increased, deposited ice exhibited four regimes: suppressed boiling, nucleate boiling, transition boiling, and the compound Leidenfrost state. The thicknesses of the compound Leidenfrost films were calculated by comparing their conductive heat transfer to their radial flow rates governed by Poiseuille’s law. The theoretical lifetime of melting Leidenfrost ice, assuming vapor/liquid layers of uniform thickness, was several times longer than that observed experimentally. We attribute this to the buoyancy of the vapor layer, which served to thin the overlying liquid film to enhance conductive heat transfer to the ice. |
Monday, November 19, 2018 12:06PM - 12:19PM |
G12.00008: Dynamics and Mechanisms of Transition Regimes to Leidenfrost State on Micro-Fabricated Surfaces Navid Saneie, Varun Kulkarni, Kamel Fezzaa, Neelesh Patankar, Sushant Anand A drop gently placed on a highly superheated surface can last for long time, running amok in different directions as it remains separated from the substrate by a vapor cushion formed due its own rapid evaporation. This regime referred to as Leidenfrost state is of great practical importance to industrial applications such as power plants, electronics cooling and boilers. Although a general trend for LFP on microstructured surfaces has previously been reported, a thorough investigation of a cross-over from transition boiling regimes to Leidenfrost state on such surfaces has not been investigated in detail. Herein, the authors discuss the role of nano/micro and hierarchical structures in controlling the Leidenfrost behavior of drops across wide temperature ranges. Precisely controlled features were obtained on substrates using photolithography techniques. high-speed imaging is used to recognize the transient boiling and rebound behavior. Theoretical and experimental analyses were conducted to map the transition regimes with respect to the surface topology. Dynamic effects due to droplet size and impact velocity were considered to elaborate on the nature and mechanism of transition boiling and Leidenfrost state. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G12.00009: Levitation of Sparkling Water Drops using Self-Generated Gas Cushion Divya Panchanathan, Philippe Bourrianne, Philippe Nicollier, Abhijatmedhi Chotrattanapituk, Kripa K Varanasi, Gareth H McKinley Previous studies have shown that droplet levitation on solid surfaces (Leidenfrost effect) can be realized by creating a vapor/air cushion under the drop using evaporation, gas perfusion or surface-driven motions. However, the major limitation of these methods is that the substrate needs to be either heat resistant, porous or mobile. A progressive loss of liquid volume is an additional drawback in the case of evaporating droplets. We demonstrate a room-temperature Leidenfrost effect that is driven spontaneously by the degassing of carbonated water droplets deposited on superhydrophobic surfaces. We observe the levitation-to-wetting transition of these degassing droplets using light interferometry on transparent superhydrophobic substrates. We characterize the timescales of the wetting transitions with respect to the concentration of dissolved carbon dioxide and show that a minimum critical dissolved carbon dioxide concentration of ≈10 mM is required for achieving droplet levitation. We finally display the practical utility of this phenomena for liquid-solid friction reduction, droplet sorting, self-propulsion, and on-demand droplet levitation using chemical reactions. |
Monday, November 19, 2018 12:32PM - 12:45PM |
G12.00010: Experimental study on pressure balance of a droplet levitating over a moving wall Yoshiyuki Tagawa, Ayumi Matsuda, Erina Sawaguchi We experimentally investigate droplet levitation over a moving glass wall in a steady state. The droplet is supported by the lubrication pressure in an air film of µm thickness between the moving wall and the droplet. The previous theoretical consideration indicated that the lubrication pressure and the bottom shape of the deformable droplet are mutually determined. However, there is little experimental validation of the pressure balance on the droplet surface, which is essential in levitating droplet problems. To verify pressure balance experimentally, this study measures three-dimensional shape of the air film, including mean curvature distribution of the surface, and calculate pressure distribution of the levitating droplet. Remarkably the lubrication pressure is found to balance with the surface tension and hydrostatic pressure. We also verify that the lift force, integral of the lubrication pressure, balances with the gravity force acting on the droplet. In addition, we provide new information based on the investigation of several parameters, especially the droplet viscosity varied over two orders magnitude. |
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