Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L12: Drops, Bubbles and Interfaces I |
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Sponsoring Units: DFD Chair: Siddartha Das, University of Maryland Room: 271 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L12.00001: Dropwise Condensation Enhancement on Geometric Features Yajing Zhao, Daniel J. Preston, Zhengmao Lu, Evelyn N. Wang Dropwise condensation, which has been demonstrated to exhibit a 5-7X higher heat transfer coefficient compared with state-of-the-art filmwise condensation, contributes to energy savings in a wide range of applications such as desalination systems, steam cycles and dew harvesting. In order to enhance dropwise condensation performance, previous studies have investigated the effects of surface geometric features on droplet growth rates and found that bumps protruding from surfaces can effectively promote dropwise condensation. In this work, we show that while bumps on surfaces enable droplets to grow faster in some cases, there are also cases where bumps on surfaces actually degrade dropwise condensation. We numerically simulated and experimentally demonstrated that even the same surface geometric feature can exert completely opposite effects on dropwise condensation of water under two different working conditions (pure vapor vs. air vapor mixture). This phenomenon is explained by comparing the heat and mass transfer resistance of the surface structure to that of the vapor transport during dropwise condensation. We expect that the fundamental understanding developed in this study will provide useful guidelines for relevant condensation applications. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L12.00002: Optimal Design of Slippery Liquid-Infused Porous Surfaces for Enhanced Condensation of Low Surface Tension Fluids Daniel J Preston, Zhengmao Lu, Yajing Zhao, Dion Antao, Kyle Wilke, Evelyn N Wang Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Dropwise condensation, where discrete droplets form on the condenser surface, exhibits 5 -- 7x higher heat transfer performance than filmwise condensation, where the condensate spreads over the surface. However, promoting dropwise condensation of low surface tension fluids is particularly challenging since the typical hydrophobic condenser coatings used to promote dropwise condensation of water (surface tension 73 mN/m) often do not repel fluids with low surface tensions (\textless 30 mN/m). Recent work has indicated that slippery liquid-infused porous surfaces (SLIPS) can promote dropwise condensation of low surface tension fluids by introducing a lubricant immiscible with the condensate into a rough structure on the condenser surface. We developed a detailed model of condensation on SLIPS using the van Oss-Chaudhury-Good theory as a framework to determine the feasibility of any arbitrary solid-lubricant-condensate system, and we validated our model with experimental results. This work enables optimal design of SLIPS for enhanced condensation of low surface tension fluids which promises significant energy savings in applications such as thermal management and power generation. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L12.00003: Capillary Surface in Micropillar Arrays Zhengmao Lu, Daniel Preston, Dion Antao, Yangying Zhu, Evelyn Wang Many microfluidic and thermal management devices utilize microstructures for tuning surface wettability and capillary transport. Prediction of the liquid-gas interface shape within the microstructures can facilitate understanding of wetting regime transitions, liquid transport, and the criteria for liquid dry-out in such applications. We developed a theoretical model that directly solves for the interface shape in micropillar arrays as a function of the interfacial pressure difference. Our simulations indicate that three regimes exist as the interfacial pressure difference varies: (I) a fully pinned mode, (II) a partially receding mode where segments of the contact line recede while the rest is still pinned, and (III) a fully receding mode where the maximum interface curvature is reached. We observed all three regimes with water and silicon micropillar arrays using an environmental scanning electron microscope. We also compared results from our current method to the energy minimization approach using Surface Evolver simulations, which shows good agreement. This work offers fundamental insights into capillary surfaces in microstructures and assists in the design of micropillar-based superhydrophobic and microfluidic devices. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L12.00004: Spontaneous elastocapillary deformations driving the formation of 2D microcoils Adam Fortais, Kathleen Charlesworth, Rafael D Schulman, Kari Dalnoki-Veress We report on the elastocapillary deformation of flexible microfibers in contact with bubbles trapped at the surface of a liquid bath. The elastocapillary interaction results in stunning 2-dimensional microfiber coils. Microfibers placed on top of bubbles are found to migrate to and wrap around the perimeter of the deformed liquid surface for certain bubble-fiber size combinations. A simple model incorporating surface and bending energies captures the spontaneous winding process. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L12.00005: Liquid-liquid separation in high porosity inverse opals Soeren Brandt, Joanna Aizenberg Oil-water mixtures pose significant industrial and environmental challenges in wastewater treatment. Submicron porosity membranes could provide the foundation of new low-cost separation techniques as local nucleation surfaces. However, membrane separation faces major roadblocks: aging of the membrane through surface fouling and pore blockage limits long-term applicability, and there is a general lack of understanding of the mechanism of separation. Inverse opals provide a structured, porous material with a reproducible, highly interconnected, bimodal pore structure that exhibits unique wetting behavior, based on which we can study the separation of oil-water mixtures in a thin, porous film using fluorescence microscopy. To understand the mechanism of liquid-liquid separation in inverse opals, we have devised an integrated microfluidic system to observe Darcy flow along the principal directions of the porous lattice. We find the pressure drop associated with surface drag to be small, and demonstrate control over the capillary pressure associated with infiltration of the porous matrix by changing the local surface chemistry. Additionally, we are able to achieve de-emulsification at low flow velocities regardless of surface chemistry. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L12.00006: Steady streaming created by a bunch of acoustically excited bubbles in microfluidics Thomas Combriat, Pierre Thibault, Philippe Marmottant Thanks to the non-linearity of Navier-Stokes equation, objects vibrating with high-amplitude in a fluid can produce a steady flow called streaming. For sufficiently high amplitudes and/or frequencies of vibration, this phenomenon holds in microfluidic systems despite usually low Reynolds number flows. We present here the steady streaming produced by acoustically excited bubbles in such conditions: thanks to a coupling mechanism, a cluster of bubbles can produce long-range and fast steady flows. This is due to the apparition of a translational mode in addition to a pulsating mode. The present experimental study evidence of a rich variety of flows produced by this system in agreement with the theory developed by F. Mekki-Berrada et al published in the Journal of Fluid Mechanics in 2016. In particular, we observed interesting patterns, with closed recirculations around bubbles in which a portion of the fluid remains trapped. Since this exclusion zones may persist even under incoming flow and as long as the acoustical excitation is present, they may prove useful for the contactless trapping of chemical compounds, allowing to switch solvent or bring new reactants. More generally, this study shows the ability of acoustically driven bubbles to create high velocity flows at microscale. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L12.00007: Experiments and lattice Boltzmann simulations for off-centered droplet collisions. Kuo-Long Pan, Matthew Andrew, Kuan-Ling Huang, Julia Yeomans Three-dimensional lattice Boltzmann simulations have been performed to investigate the flow field underlying the impact dynamics of two identical droplets. The symmetry of the head-on droplet collision is broken by offsetting the approaching trajectories of the droplets. The outcomes are determined by three factors: the Weber number which measures the impact inertia relative to the surface tension, the Ohnesorge number, which characterizes the viscous force relative to surface force, and the angle between the colliding paths. The results are compared to experimental sequences obtained by a drop-on-demand technique with high-speed imaging process. Various mechanisms leading to separation of coalesced droplets are investigated and analyzed computationally, including reflexive separation, rotational separation and stretching separation. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L12.00008: Walkers in cavities Boris Filoux, Maxime Hubert, Peter Schlagheck, Nicolas Vandewalle When a droplet is placed onto a vertically vibrated bath, it can bounce without coalescing. Upon an increase of the forcing acceleration, the droplet is propelled by the wave it generates and becomes a walker with a well defined speed. Recently, some 2d confining systems for walking droplets have been developed: cylindrical cavity, harmonic potential or the use of Coriolis force. In addition, the interactions between two identical walkers have been studied in a 2d case. Nevertheless, no study focuses on 1d dynamics and their properties. In this work, we show it is possible to confine a walker in a quasi mono-dimensional geometry by using submerged cavities. We focus on the interactions between droplets. Then, we study the speed of a pair of walkers and show that the distance between the drops affects the group speed. The closer the drops are, the faster they move. We also propose a numerical model to characterize the distance quantization, and the evolution of the speed of a string of droplets. We investigate the case of a string of droplets, and discuss the influence of the number of droplets and the distance between them on the string speed. We show that the drops share a coherent wave. Finally, we discuss the influence of the memory parameter. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L12.00009: Non-equilibrium phase stabilization versus bubble nucleation at a nanoscale-curved Interface Jarrod Schiffbauer, Tengfei Luo Using continuum dynamic van der Waals theory in a radial 1D geometry with a Lennard-Jones fluid model, we investigate the nature of vapor bubble nucleation near a heated, nanoscale-curved convex interface. Vapor bubble nucleation and growth are observed for interfaces with sufficiently large radius of curvature while phase stabilization of a superheated fluid layer occurs at interfaces with smaller radius. The hypothesis that the high Laplace pressure required for stable equilibrium of very small bubbles is responsible for phase stability is tested by effectively varying the parameter which controls liquid-vapor surface tension. In doing so, the liquid-vapor surface tension-- hence Laplace pressure--is shown to have limited effect on phase stabilization vs. bubble nucleation. However, the strong dependence of nucleation on leading-order momentum transport, i.e. viscous dissipation, near the heated inner surface is demonstrated. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L12.00010: Capillary deposition of advected particles Emilie Dressaire, Aymeric Debaisieux, Federico Gregori, Alban Sauret The deposition and aggregation of particles flowing through a confined environment can dramatically hinder the transport of suspensions. Yet, the mechanisms responsible for the deposition of particles in shear flow are not fully understood. Here, we use a macroscopic model system in which the attractive interactions are due to capillary effects. Floating particles are advected on the surface of a water channel and their two-dimensional trajectories can be modified by fixed obstacles. By varying the flow rate of the liquid, the wetting properties and size of the particles and obstacles, we can tune the magnitude of the capillary and hydrodynamic forces that determine the probability of capture and the position on the obstacle. We compare our results with a theoretical model that captures the trajectory of the particle before it reaches the obstacle and accounts for the inertia of the particle to determine the equilibrium position on the obstacle. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L12.00011: Droplets on bent fibers Floriane Weyer, Zhao Pan, William Pitt, Tadd Truscott, Nicolas Vandewalle Droplets on fibers are part of our everyday lives. Many phenomena involve drops and fibers such as the formation of dew droplets on a spiderweb, the trapping of water droplets on cactus spines or the motion of droplets on wetted moss hairs. These topics have been widely studied. In particular, Lorenceau \textit{et al.} \footnote{Lorenceau \'{E}, Clanet C, Qu\'{e}r\'{e} D (2004) Capturing drops with a thin fiber. \textit{Journal of colloid and interface science} 279(1):192–197} determined the critical volume of a water droplet hanging on a horizontal fiber. Here, we address a similar question : we try to find out the maximum droplet size on bent fibers, which are able to hold significantly more water than horizontal fibers. Indeed, we noticed that, in nature, some specific plants can hold large rain droplets thanks to their Y-shaped leaves. We try to mimic these structures with nylon fibers, of different diameters, bent with various angles. For each set-up, the critical water volume is determined. Finally, we propose models of the physics involved in determining droplet size that could be implemented in future fiber-based microfluidic devices. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L12.00012: Elastocapillary bending of microfibers around liquid droplets Kari Dalnoki-Veress, Rafael Schulman, Amir Porat, Kathleen Charlesworth, Adam Fortais, Thomas Salez, Elie Raphael We present a study of the elastocapillary deformation of flexible microfibers in contact with liquid droplets. As the size of the contacting droplets increases, fibers are found to bend more in response. Finally, at a critical droplet size, proportional to the bending elastocapillary length, the fiber spontaneously winds itself around the droplet. Simple theoretical considerations yield predictions which are in agreement with the experimental results. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L12.00013: Droplets act as compass needles for the tension in a membrane Rafael Schulman, Rene Ledesma-Alonso, Thomas Salez, Elie Raphael, Kari Dalnoki-Veress We present experiments which study droplets atop thin elastomeric films with anisotropic tension. Surprisingly, we find that the droplets are not spherical caps and become elongated along the axis of highest tension. As such, liquid droplets create a map for the principal stress directions in a film. In our experiments, we completely determine the contact line geometry using a combination of contact angle measurements and optical profilometry. In addition, we measure an out-of-plane deformation of the film surrounding the droplet. Simple theoretical arguments successfully capture the experimental findings. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L12.00014: Capillary trapping in thin-film flows of particles Alban Sauret, Michael Gomez, Emilie Dressaire Flows of suspensions have been modeled on a continuum level by using constitutive relations to capture how the viscosity varies with the particle concentration. However, in thin liquid films, where the thickness of the liquid layer is comparable to the particle size, the particles deform the liquid interface, which leads to local interactions. These effects modify the transport of particles and could result in the contamination of the surface and the loss of transported material. Here, we characterize how capillary interactions affect the transport and deposition of non-Brownian particles moving in thin liquid films. We focus on gravitational drainage flows, in which the film thickness becomes comparable to the particle size. Depending on the concentration of particles, we find that the dynamics of the drainage exhibits behavior that cannot be captured with a Newtonian model, due to the deposition of particles on the substrate. [Preview Abstract] |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L12.00015: Diffusion of Water Nanodroplets on MoS$_{\mathrm{2}}$ Monolayer Beibei Wang, Rajiv Kalia, Priya Vashishta, Aiichiro Nakano Diffusion of water (H$_{\mathrm{2}}$O) on the molybdenum disulfide (MoS$_{\mathrm{2}})$ surface has recently attracted a great deal of attention. We have performed molecular dynamics (MD) simulation to study the diffusion of water nanodroplets on a monolayer of MoS$_{\mathrm{2}}$. Our simulation reveals that water nanodroplets diffuse rapidly on the MoS$_{\mathrm{2}}$ monolayer and the diffusion coefficient is inversely proportional to the nanodroplet radius. We also find that the H$_{\mathrm{2}}$O molecular distribution at the H$_{\mathrm{2}}$O-MoS$_{\mathrm{2}}$ interface is akin to the MoS$_{\mathrm{2}}$ layered structure and that friction coefficients of nanodroplets are small at the MoS$_{\mathrm{2}}$ surface. [Preview Abstract] |
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