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 LW: Surface Tension III |
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Chair: Alex Lukyanov, University of Reading, UK Room: Hyatt Regency Long Beach Regency C |
Monday, November 22, 2010 3:35PM - 3:48PM |
LW.00001: Capillary rise in wedges Alexandre Ponomarenko, Christophe Clanet, David Quere A wetting liquid put into contact with a thin vertical tube rises spontaneously in it, reaching a final height $z=h_{e}$ given by Jurin's law: $\frac{h_{e}}{r}=2\left(\frac{a}{r}\right)^2cos\theta_{c}$ where $r$ is the radius of the tube, $a=\sqrt{\frac{\gamma}{\rho g}}$ is the capillary length, based on the liquid surface tension $\gamma$, liquid density $\rho$ and gravity $g$, and $\theta_{c}$ is the contact angle characterizing the wetting of the liquid on the solid.--- Also, when $z\ll h_{e}$, where gravity can be neglected, the front of the liquid follows Washburn's law: $z=\sqrt{2\frac{\gamma r cos\theta_{c}}{\eta}t}$, where $\eta$ is the liquid viscosity.--- This works for all systems having a ``closed'' geometry, that is a scaling length, provided this scaling length is smaller than $a$.--- We use systems of ``open'' geometry, without scaling length, typically wedges with different geometries and show both experimentally and theoretically that the meniscus rises following the universal law : $\frac{h(t)}{a}\sim(\frac{\gamma}{\eta a}t)^{1/3}$. It differs from the case of ``closed'' geometry because it rises indefinitely and with a different dynamic. It is universal in the sense that it does not depend on the special geometry of the wedge. [Preview Abstract] |
Monday, November 22, 2010 3:48PM - 4:01PM |
LW.00002: Delayed Coalescence of Sessile Droplets with Different but Miscible Liquids Stefan Karpitschka, Hans Riegler Due to capillary forces, two sessile droplets of miscible liquids will fuse when they contact each other. Usually the droplet fusion proceeds very fast, delayed mostly by viscous forces. However, quite unexpected, it was observed recently\footnote{H. Riegler, P. Lazar, \textit{Langmuir} \textbf {24}, 6395 (2008).} that the coalescence of sessile droplets of completely miscible liquids can be delayed up to minutes. After a first contact, the droplets remain separated by a thin liquid neck and push each other across the substrate before they finally merge. The delayed coalescence may be highly relevant for technical applications, for instance in the sector of semiconductor manufacturing, where controlled contact line displacement is a key technology. It is assumed that the coalescence is delayed by a Marangoni convection through the thin film connecting the drops. This suggests that the effect is quite common. A sharp transition from fast to delayed coalescence was found\footnote{S. Karpitschka, H. Riegler, \textit{Langmuir} \textbf{26}, 11823 (2010).} as the difference in surface tensions exceeded $\approx 4\,\rm{mN/m}$, irrespective of other liquid properties like absolute surface tensions or viscosities. We present new experimental results addressing the dynamics of the liquid neck between the drops from which we can distinguish various coalescence modes. [Preview Abstract] |
Monday, November 22, 2010 4:01PM - 4:14PM |
LW.00003: Transition Regimes of Jet Impingement on Rib and Cavity Superhydrophobic Surfaces Michael Johnson, Daniel Maynes, Brent Webb We report experimental results characterizing the dynamics of a liquid jet impinging normally on superhydrophobic surfaces spanning the Weber number (based on the jet velocity and diameter) range from 100 to 2000.The superhydrophobic surfaces are fabricated with both silicon and PDMS surfaces and exhibit micro-ribs and cavities coated with a hydrophobic coating. In general, the hydraulic jump exhibits an elliptical shape with the major axis being aligned parallel to the ribs, concomitant with the frictional resistance being smaller in the parallel direction than in the transverse direction. When the water depth downstream of the jump was imposed at a predetermined value, the major and minor axis of the jump increased with decreasing water depth, following classical hydraulic jump behavior. When no water depth was imposed, a regime change was observed within the Weber number range explained. For We $<$ 1200, the flow forms a filament at the edge of the ellipse, where the flow moves along the rim of the ellipse toward the major axis. The filaments then join and continue to move parallel to the ribs. For 1200 $<$ We $<$ 1800, the filaments beyond the ellipse break into multiple streams and droplets and begin to take on a component perpendicular to the ribs. For We $>$ 1800 a small amount of water flows purely in the transverse direction. [Preview Abstract] |
Monday, November 22, 2010 4:14PM - 4:27PM |
LW.00004: A Numerical Method for Variable Surface Tension Effects in Non-Isothermal Atomization with Overset Grids Peter Brady, Juan Lopez, Marcus Herrmann Overset grids lend themselves to problems requiring very high resolution in a region of the domain. Here we aim to couple an overset-grid method to our flow solver and interface tracking scheme for the purpose of high resolution atomization simulations. Often atomization occurs in non-isothermal environments, such as in combustion devices. In these devices, thermal fluctuations can be significant on length scales associated with the liquid atomization process. Since the surface tension force is a function of local temperature, these thermal fluctuations can result in large local variations of the surface tension force, thereby potentially significantly impacting the details of the atomization process. To handle these potentially large thermal fluctuations we introduce an overset grid in the vicinity of the interface. This allows us to resolve the complex thermal boundary layers that can develop without the expense of refining the flow solver grid. The Refined Level Set Grid (RLSG) method is used to track the liquid/gas phase interface using an auxiliary high resolution equidistant Cartesian grid. Verification tests using the method of manufactured solutions for multi-phase flow will be presented. [Preview Abstract] |
Monday, November 22, 2010 4:27PM - 4:40PM |
LW.00005: An Experimental Investigation of the Theory of Electrostatic Deflections Javed Siddique, Robert Deaton, Eric Sabo, John Pelesko The so-called ``pull-in'' instability is a ubiquitous feature of electrostatic actuation. In systems where an applied voltage is used to actuate or move mechanical components, it is observed that when the applied voltage exceeds a critical value, electrostatic forces become dominant over elastic forces and the mechanical components ``pull-in'' or collapse into one another. This study of this instability is particularly relevant in the field of microelectromechanical and nanoelectromechanical systems (MEMS \& NEMS), where electrostatic actuation is often used. This instability severely restricts the design space of such systems. Here, key theoretical results concerning this instability are surveyed and compared to a new experimental study of electrostatic deflections. Gaps between theory and experiment are uncovered and directions for future modeling and analysis indicated. [Preview Abstract] |
Monday, November 22, 2010 4:40PM - 4:53PM |
LW.00006: Ant Tower Nathan Mlot, Sho Shinotsuka, David Hu Ants walk via adhesive drops of fluid extruded by their feet. They also use these drops as mortar to build structures such as rafts, bridges and towers, each composed of thousands of ants linked together. We investigate experimentally the construction of triangular ant towers braced by hydrophobic walls. Particular attention is paid to the relationship between tower height and contact angle hysteresis of the wall. We rationalize tower height according to ant adhesion, and tower shape according to the constraints on a column of constant strength. [Preview Abstract] |
Monday, November 22, 2010 4:53PM - 5:06PM |
LW.00007: Wet-dog shake Andrew Dickerson, Zack Mills, David Hu The drying of wet fur is a critical to mammalian heat regulation. We investigate experimentally the ability of hirsute animals to rapidly oscillate their bodies to shed water droplets, nature's analogy to the spin cycle of a washing machine. High-speed videography and fur-particle tracking is employed to determine the angular position of the animal's shoulder skin as a function of time. We determine conditions for drop ejection by considering the balance of surface tension and centripetal forces on drops adhering to the animal. Particular attention is paid to rationalizing the relationship between animal size and oscillation frequency required to self-dry. [Preview Abstract] |
Monday, November 22, 2010 5:06PM - 5:19PM |
LW.00008: Elastocapillary imbibition Camille Duprat, Jeffrey Aristoff, Howard Stone The deformation of flexible structures under capillary forces (elastocapillarity) is relevant to many biological and engineering processes and has been the subject of several recent studies. Here, we focus on the capillary filling, or imbibition, of a gap between flexible boundaries. We examine two model systems of elastocapillary imbibition using a combination of experiment, theory and numerical simulation. In each case we determine the time to reach equilibrium (if one exists) and establish a criterion for coalescence of the elastic boundaries. [Preview Abstract] |
Monday, November 22, 2010 5:19PM - 5:32PM |
LW.00009: Capillary rafts and their destabilization Suzie Protiere, Manouk Abkarian, Jeffrey Aristoff, Howard Stone Small objects trapped at an interface are very common in Nature (insects walking on water, ant rafts, bubbles or pollen at the water-air interface, membranes...) and are found in many multiphase industrial processes. The study of such particle-laden interfaces is therefore of practical as well as fundamental importance. Here we report experiments on the self-assembly of spherical particles into capillary rafts at an oil-water interface and elucidate how such rafts sink. We characterize different types of sinking behavior and show that it is possible to obtain ``armored droplets,'' whereby the sinking oil is encapsulated within a shell of particles. [Preview Abstract] |
Monday, November 22, 2010 5:32PM - 5:45PM |
LW.00010: Boiling Phenomena of Capillary Flow in Heat Pipe Applications Conan Zhang, Domagoj Jursic, Kaitlyn Hunt, Carlos Hidrovo Boiling is scarcely documented in heat pipe literature due to the complexity of the phenomena. Most heat pipe designers simply avoid the boiling limit and design systems that operate well below boiling. However, with recent technological advances and limited development of heat pipe technology, heat pipes are encountering higher temperatures. Consequently, it is important to understand the boiling phenomena associated with higher temperatures. This work presents experimental data at various heat fluxes and surface temperatures for micropillar wicks with different dimensions. The empirical data is evaluated against earlier theoretical work that predicts the onset of the capillary and boiling limits and their corresponding liquid flow rates for a given heat flux. Any disruption in capillary action caused by boiling can be identified by correlating the flow rate of the fluid through the porous structure with the surface temperature. When boiling occurs, the flow rate can be observed to deviate from expected values and the temperature increases substantially. Visual diagnostics via time lapse photography was also utilized to help visualize the fluid film at the different heat flux ranges. [Preview Abstract] |
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