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 A11: Drops: Wetting and Spreading I |
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Chair: Satish Kumar, University of Minnesota, Twin Cities Room: Georgia World Congress Center B216 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A11.00001: Effect of Evaporation Rate on Stick-Slip Behavior of the Contact Line Sahar Andalib, Pirouz Kavehpour Droplet evaporation has a lot of industrial applications such as spray cooling, coating technology, and evaporative self-assembly. During evaporation of a drop on solid surface, maximum evaporation rate occurs at the contact line. Hence, the mechanism close to the contact line highly controls the process of the evaporation such as total evaporation time, pinning. Despite its importance, this mechanism is not yet fully understood. Theoretical and experimental studies focused on microscopic behavior of contact line have shown the dependence of contact angle on the rate of evaporation. Macroscopic theoretical studies have shown different behavior of the contact angle and droplet base radius for the case of strong and weak evaporation. A typical droplet evaporation consists of two well-defined stages. Constant radius mode occurs initially which is followed by a stick-slip mode. Stick-slip behavior of the contact line is a consequence of competition between unbalanced Young’s force and adhesion forces due to roughness. The present work is an experimental investigation of the effect of evaporation rate on macroscopic behavior of the contact line in case of stick-slip. This study contributes to the experimental knowledge of the physics of evaporation at macroscopic scale. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A11.00002: Predicting the Dynamics of Wetting from the Fluctuations of Forces at an Equilibrium Solid-Liquid Interface Joel De Coninck, Juan Carlos Fernandez Toledano, Terry Blake We use molecular dynamics (MD) and Lennard-Jones potentials to model a liquid bridge between two solid surfaces and apply the fluctuation-dissipation theorem to compute the friction between the liquid and the solid walls at equilibrium. We find that the frictions obtained per unit length of the interface are the same as the coefficients of contact-line friction found by applying the molecular-kinetic theory (MKT) of dynamic wetting to the dynamic contact angles found in MD simulations of spreading liquid drops and Couette flow in a liquid bridge using identical methods and potentials. Our results show the same dependence on the strength of solid-liquid interactions and the equilibrium contact angle. Combined with the lattice spacing of the solid surface, the results allow one to extract the key MKT parameters necessary to predict the full velocity-dependence of the microscopic dynamic contact angle. These findings confirm a common underlying mechanism for contact-line friction and slip at solid liquid interfaces. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A11.00003: Sticky Slippy Hemi-solids Saurabh Nath, Armelle Keiser, Christophe Clanet, David Quéré A textured solid infused with a liquid comprises a class of materials in between a liquid and a solid. These hemi-solids may exhibit very little adhesion due to the lack of pinning sites. Here we show that alongside being `slippery’, these surfaces can be `sticky‘ as well. We investigate this by taking an elementary system consisting of a millimetric water droplet placed between two surfaces, infused with oils of different viscosities. We find that if the upper plate viscosity is greater, then by pulling the upper plate at sufficiently high velocities it is possible to detach the droplet completely off the bottom plate. The captured droplet can be subsequently deposited on the lower plate by bringing it in contact and pulling the plates apart slowly. In this talk, we will discuss the physics of such a viscous tweezer and the parameters that govern it. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A11.00004: Spreading, fingering, and deposition in volatile liquid mixtures Asher Mouat, Clay Wood, Justin E. Pye, Justin C. Burton When a volatile liquid drop is placed on a wetting surface, it rapidly spreads and forms a circular, thin film before evaporating. Surprisingly, when even trace amounts (~0.05%) of a less volatile liquid is present in the volatile liquid, the contact line destabilizes, leading to the formation of structures resembling “fingers” or “pearls”. We have characterized this phenomena using isopropanol with various contaminants (ethylene glycol, glycerol, dodecane, water, and acetone) on surfaces of varying wettability. Although pearls always form due to enhanced evaporation at the contact line, we find that fingers only form when the contaminant has a higher surface tension than the isopropanol, and partially wets the surface. The characteristic size of the structures increases with contaminant concentration. In addition, we find that the local vapor pressure of the isopropanol strongly affects the wetting characteristics of the contaminant. Once the isopropanol evaporates, striking droplet patterns of the contaminant are left behind. Some resemble crystalline lattices of various drop sizes, or the contaminant liquid may form a sub-micron thin film. These highlight the role of trace impurities in volatile liquids, and the deposition patterns they leave behind. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A11.00005: Dynamics of viscous adhesion : From fingering instability to capillary bridges Manon L'Estimé, José Bico, Etienne Reyssat, Ludovic Keiser The adhesion between solid surfaces coated with a viscous liquid is limited by the propagation of a capillary bridge through the narrow gap separating both surfaces. Although the viscous adhering liquid pushes air of low viscosity, the adhesion front undergoes, in some situations, a fingering instability. What are the ingredients involved in the adhesion dynamics and in this unexpected instability? To address these questions, we consider a simplified model experiment where a solid beam is placed horizontally above a bath of liquid at a small distance in comparison with the capillary length. As a liquid bridge is created at one end of the beam, the bridge spontaneously expands towards the opposite extremity. We first show the strong dependence of the meniscus velocity with the liquid properties, the separation gap and the thickness of the reservoir. We then investigate the interactions between bridges propagating along parallel beams. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A11.00006: Early life of a liquid bridge Adel Djellouli, Shmuel M Rubinstein The formation, stretching and breakup of liquid columns have concentrated a considerable amount of work for more than a century, since the classic experimental work of Plateau (1863) and the theoretical work of Young (1805), Laplace (1805) and later Rayleigh (1879). This continuous interest is motivated by the ubiquity of these structures in nature and industrial processes such as most printing and coating techniques and flow, evaporation, and condensation in porous media. With the rise of high speed imaging, it is possible to look at the early stages of liquid-solid contact and investigate the nucleation of a liquid bridge. We propose to use a 1D setup where a droplet is deposited on a moving plate and approached to a fixed transparent plate, by using a linear stage capable of achieving micro-steps. To image the contact between the droplet and the fixed plate, we use Total Internal Reflection (TIR) technique [Rubinstein 2004] where we shine mono-chromatic light from the side of the plate in a way that only lets an exponentially decaying evanescent field escape at the immediate vicinity of the surface [Kolinski 2012]. When the liquid makes contact with the surface, the region of contact illuminates and we are able to measure its evolution with time, as it spreads. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A11.00007: Reversible Motion of a Contact Line Audrey Profeta, Esmeralda Orozco, Juan A. Ortiz Salazar, Jeanette Smit, Brian Kroger, Aidan McGuckin, Dani Medina, Nathan C. Keim We study the evolution of the liquid-solid-vapor contact line of water held in a narrow gap between two acrylic plates. A syringe pump injects and withdraws a constant, small volume of the water, driving the contact line back and forth repeatedly and changing its shape. We take photos of the contact line after each cycle. Comparing these images to each other, we find that below a critical value of infused volume, after several cycles the contact line reaches a steady state in which it always returns to the same shape, despite depinning and executing many small jumps during its motion. Above that value the shape fluctuates in the steady state. This suggests a transition reminiscent of those seen in other systems including cyclically deformed particle suspensions and solids. We discuss possible explanations in terms of the microscopic dynamics of the contact line. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A11.00008: Universal and non-universal hole closure in a liquid layer Gunnar Peng, John Lister We study how a thin layer of viscous fluid on a solid substrate flows inward to fill an initially dry circular region, under the action either of surface tension on the free surface or of stresses in a thin elastic sheet covering the fluid. We solve the lubrication equations numerically in various scenarios, including the closure of a hole in an infinite film, the release of an annular volume of fluid, and the prying apart of two elastic discs. For surface tension (or an elastic membrane), as the hole shrinks the closing speed diverges rapidly and a universal solution is obtained that does not depend on the initial or far-field conditions and is described by approximate power laws with slowly varying exponents depending on the length scale of the contact-line regularization. For elastic bending, the closing speed diverges only logarithmically, and the closure behaviour is non-universal as it depends on the initial and/or far-field boundary conditions. We elucidate the main physical balances with an asymptotic analysis involving a multitude of nested boundary layers. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A11.00009: Modeling of an oscillating liquid bridge and its energy harvesting: Simulation and Experiment Kyung Chun Kim, DAEYEON KIM In order to carry out parametric studies on the Reverse-Electro-Wetting on Dielectric (REWOD) phenomenon, which has been attracting attention as a part of energy harvesting technology in recent years, basic research on the physical behavior analysis of vibrating liquid-bridge is essential. In this study, a liquid droplet oscillating between two plates is visualized and the voltage output value was measured by REWOD. For the upper plate, hydrophobic surface treated by PTFE is used and the lower plate is tested using its hydrophilic surface properties of ITO glass. A modeling study was carried out on the dynamic behavior of vibrating liquid-bridge between two flat plates using the phase field method based on the finite element method. For incompressible fluids, the Navier-Stokes equation including gravity force and surface tension force is employed. In comparison with the contact area measured in the experiment, the analytical results agree with each other within 10% error range. Furthermore, experimental and analytical studies are carried out on the REWOD characteristics of the mixture of glycerol and pure water at different concentrations. |
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