Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session AK: Free Surface Flows: Spreading and Wetting |
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Chair: Gustav Amberg, KTH Stockholm Room: Long Beach Convention Center 201B |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AK.00001: Surfactant-assisted spreading of an emulsion Zhenzhen Li, Matthieu Roche, Arnaud Saint-Jalmes, Howard A. Stone We studied experimentally the spreading dynamics of a drop of a surfactant-stabilized oil-in-water emulsion over the free surface of a layer of a solution of the same surfactant. The dynamics display three regimes. After the deposition of the emulsion drop, oil droplets are advected by a Marangoni flow, due to the difference in surfactant concentration between the emulsion and the liquid surface, and spread at the air/liquid interface. The oil droplets eventually stop, forming a dense ring, whose diameter is constant as long as oil droplets are transported by Marangoni flow. During this stage, oil droplets are moving rapidly on a surface with a low droplet concentration. Once the initial drop is empty, the ring collapses on itself, a phenomenon not yet reported experimentally to our knowledge. Spreading and retraction occur on a few hundred milliseconds while the ring stage lasts a few seconds. Using a laser sheet reflected by the surface of the liquid layer, we measured the shape of the surface and identified a jump of a few hundred microns in the layer thickness at the location of the ring. The existence of this jump points to hydrostatic pressure as the driving stress for retraction. We also show that this system shares many features with other jumps. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AK.00002: Quasi-Steady Capillary Driven Flow in Complex Geometries Mark Weislogel, Alex Baker, Donald Pettit Lubrication theory has been successfully applied to a large class of local capillary driven flows along interior corners in simple conduits of uniform cross section (i.e. right polygonal cylinders). In this work, the evolution equations governing the local corner flows are re-scaled based on global conduit geometry, and the resulting system of equations are solved analytically in the quasi-steady limit. Several important closed form solutions are obtained with applications to passive filling, draining, and phase separations at both micro and macro length scales. Simple experiments are conducted that confirm the essential assumptions of the approach. As an example of the utility of the solutions, optimal geometries are computed for a unique ``large length scale'' flow aboard a spacecraft---a microgravity coffee cup. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AK.00003: How cats lap Roman Stocker, Pedro Reis, Sunghwan Jung, Jeffrey Aristoff We studied the lapping of the domestic cat ({\it Felis catus}) by combining high-speed photography with a laboratory model of lapping. We found that {\it Felis catus} laps by a subtle mechanism based on water adhesion to the dorsal side of the tongue and the creation of a liquid column, exploiting inertia to defeat gravity and pull liquid into the mouth. The competition between inertia and gravity controls the pinch-off time of the column, determining the optimal lapping frequency, $f$. {\it Felis catus} was found to operate near the optimum and theoretical analysis yielded a scaling, $f \sim M^{-1/6}$, of lapping frequency with animal mass, $M$. This prediction was verified by measuring lapping frequency across felids, from ocelots to lions, suggesting that the lapping mechanism is conserved among felines. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AK.00004: Thermocapillary-assisted pulling of free liquid films Benoit Scheid, Ernst van Nierop, Howard Stone We study the formation of a free liquid film that is pulled out of a bath at constant speed and stabilized by the action of thermocapillary stresses prescribed at the free surfaces. We show that only large shear induced by thermocapillary stresses allows for the stable pulling of the liquid film and that both extensional viscous stresses and gravity play no role. For small speeds and negligible inertia, the resulting thickness of the free film in independent on the pulling speed and proportional to the capillary length $lc$ as well as to a parameter $\Gamma$ that measures the relative amplitude of the surface tension change at the interface. If this change is imposed (through a temperature gradient) along a distance $d$ larger than the characteristic length $\ell=lc\sqrt{2\Gamma}$ of the system, the film thickness decreases with increasing $d$; otherwise it is independent of $d$. For large speeds and non-negligible inertia, the film thickness decreases with an increase of the Weber number. We also show how the results depend on heat transfer properties. The present theory suggests that very thin ribbons or foils of molten material can be drawn out of a melt over a wide range of thicknesses at speeds relevant to manufacturing. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AK.00005: Increasing the critical speed of wetting failure through meniscus confinement Eric Vandre, Satish Kumar, Marcio Carvalho Dynamic wetting is a crucial step of fluid-fluid displacement along a solid surface, such as the deposition of a coating liquid onto a moving substrate. At some critical process speed wetting fails and the displaced phase (e.g. air) is entrained within the displacing phase. Improving upon current industrial production speeds requires a better understanding of how system parameters influence wetting failure. Confinement of the wetting meniscus is one such parameter commonly found in high-speed coating methods, though its influence remains unclear. In this study, we explore the effects of confinement on wetting failure with a laboratory-scale plunge-coating system. Our experimental apparatus consists of a steel roll that plunges into a bath of glycerol. Confinement is imposed by bringing a coating die near the wetting line, and liquid is injected through the die to compensate for liquid being dragged away with the roll. Flow visualization is used to record the critical roll speed at which wetting failure occurs. The data show a clear increase in the critical speed with increasing confinement. A model based on the lubrication approximation does a remarkable job in accurately predicting the increase in the critical speed relative to the unconfined value. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AK.00006: Inertial and three-dimensional effects in stretching liquid bridges near plates and cavities Shawn Dodds, Marcio Carvalho, Satish Kumar The dynamics of liquid bridges are relevant to a wide variety of applications, including high-speed printing and extensional rheometry. Analysis of these systems is often performed assuming axisymmetric Stokes flow, although in printing processes these assumptions may not always hold. To address this issue, we use the finite element method to study the stretching of a finite volume of liquid between two surfaces in two model problems. In the first problem, we consider an axisymmetric liquid bridge between a stationary cavity and a moving flat plate. The contact lines are allowed to slip, and we evaluate the effect of the Reynolds number and contact angle on the transfer of liquid to the moving plate. For fixed contact angles, an increase in the Reynolds number leads to an increase in the liquid transfer. In the second problem, we consider a cylindrical liquid bridge with fixed contact lines between two flat plates, one of which is stationary while the other is simultaneously stretching and rotating. As the rotation rate is increased for a fixed stretching rate, the liquid transfer to the rotating plate is decreased. The mechanisms behind these observations will be discussed. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AK.00007: A general mechanism for the meandering of rivulets Nadine Valade, Adrian Daerr, Jens Eggers, Laurent Limat A rivulet flowing down an inclined or vertical plane often does not follow a straight path, but starts to meander. We show that this instability can appear from two key ingredients: fluid inertia and anisotropy of the friction between rivulet and substrate. Meandering occurs if the fluid motion normal to the instantaneous flow direction is more difficult than parallel to it. This slows down the downstream motion of a meander with respect to that of the fluid, and centrifugal effects can develope in the curved part of the stream. We give a quantitative criterion for the onset of meandering, and confirm it by comparing to the flow of a rivulet between to glass plates which are wetted completely. Above the threshold, the rivulet follows a irregular pattern with a typical wavelength of a few centimeters. This very general mechanism should hold in very different situations (inclined or vertical plates, total or partial wetting, pure fluid or surfactants...) provided that pinning effects of contact lines are not too strong. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AK.00008: Flow transition behavior between the film flow and rivulet flow on an inclined wall Yoshiyuki Iso, Xi Chen Gas-liquid two-phase flows on the wall like liquid film flows, which are the so-called wetted wall flows, are observed in many industrial processes such as absorption, desorption, distillation and others. For the optimum design of packed columns widely used in those kind of processes, the accurate predictions of the wetted wall flow behavior in packing elements are important, especially in order to enhance the mass transfer between the gas and liquid and to prevent flooding and channeling of the liquid flow. The present study focused on the effects of the change of liquid flow rate and the wall surface texture treatments on the characteristics of wetted wall flows which have the drastic flow transition between the film flow and rivulet flow. In this paper, gas-liquid two-phase flow simulation by using the volume of fluid (VOF) model is applied into wetted wall flows. Firstly, present results showed that the hysteresis of the flow transition between the film flow and rivulet flow arose against the increasing or decreasing stages of the liquid flow rate. It was supposed that this transition phenomenon depends on the history of flow pattern as the change of curvature of interphase surface which leads to the surface tension. Secondary, the present simulations showed that surface texture treatments added on the wall can improve the prevention of liquid channeling and can increase the wetted area. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AK.00009: Experimental measurements of contact angles with evaporation by interferometry Julien Sebilleau, Sam Dehaeck, Pierre Colinet Volatile liquids, on a substrate under total wetting conditions, exhibit an apparent contact angle even in the case of a static contact line. This contact angle is linked to the evaporation process that induces a (micro)flow in the contact line region. We study experimentally this contact angle for liquids evaporating into ambient air, in the case of a meniscus generated at the top of a Hele-Shaw cell, the two glass walls of which being placed at different heights. The shape of this meniscus is then recorded with two kinds of interferometers (Mach-Zehnder in transmission, and reflection interferometry), which allow an accurate measurement of the apparent contact angle at some distance from the actual contact line. Both static and moving (advancing or receding) contact lines situations are studied and several liquids are used. For advancing contact lines, instabilities leading to droplets formation are also observed [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AK.00010: Velocity measurements near a moving contact line with sub-micron resolution Bian Qian, Kenneth Breuer Employing high-speed particle tracking, we experimentally investigate the slip boundary condition in the vicinity of a moving contact line. A liquid bridge was established between a stationary hydrophobic glass slide and a rod. By translating the rod at a controlled speed, we establish a well-controlled moving contact line. The liquid was seeded, either with nano-scale fluorescent particles or with quantum dots. Evanescent wave illumination, with exponentially decaying intensity, was used for particle illumination, which allows for three dimensional measurements of the flow field near the liquid/solid interface. The motions of the contact line and the particles were captured using a high speed camera coupled to a high-resolution microscope. Slip length was extracted from the particle motions and shown to be a function of the distance to the contact line. Different behaviors were observed between advancing and receding motions of the contact line. Measurements with different-sized particles were performed to correct for tracer particle effects. [Preview Abstract] |
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