Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session JP: Surface Tension Effect I |
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Chair: Tom Mullin, University of Manchester Room: Salt Palace Convention Center 251 D |
Monday, November 19, 2007 3:35PM - 3:48PM |
JP.00001: Fluid dynamics of floating particles Nadine Aubry, Pushpendra Singh, D.D. Joseph We study the motion of micron sized particles floating on the interface between two immiscible fluids both experimentally and numerically. In our direct numerical simulations (DNS) the particles are moved respecting the fundamental equations for the motion of both the fluids and the solid particles without the use of any models. The fluid-particle motion is resolved by the method of distributed Lagrange multipliers and the interface is deformed by the method of level sets. We simulate the evolution of heavier-than-liquid hydrophobic spheres to their equilibrium depth and their transient free motions leading ultimately to self-assembly under lateral forces collectively called capillary attraction. We also investigate the influence of an externally applied electric field on the particles motion at the interface. The dependence on the parameters such as the dielectric properties of the fluids and the particles, the particles' position within the interface, and the strength of an externally applied electric field on the motion of the particles is also studied. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JP.00002: Metastability and roll-off behavior of droplets on superhydrophobic surfaces Kripa Varanasi, Nitin Bhate, Ming Hsu, Shannon Okuyama, Tao Deng, Judith Stein We studied the wetting behavior of water droplets on arrays of hydrophobic square posts. To determine the droplet wetting state, we measured static contact angles and compared the results to predictions for equilibrium Cassie and Wenzel states. These comparisons showed that there are three wetting regimes: equilibrium Cassie at small post spacing, equilibrium Wenzel for large post spacing, and an intermediate state at medium post spacing. Next, we measured droplet roll-off angle and surprisingly found that it was minimized on surfaces in the intermediate range that were expected to induce Wenzel-like wetting rather than on surfaces that exhibit equilibrium Cassie wetting. We argue that the droplets in the intermediate range are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the posts. These metastable Cassie droplets showed superior roll-off behavior because the effective length of the contact line that is pinned to the surface is reduced. We have developed models that can predict this roll-off behavior and the transition between the metastable Cassie and Wenzel regimes. This fundamental understanding can be used to optimize texture design for properties such as water repellency. [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JP.00003: Effects of length scale on determining surface dilatational viscosity Jonathan Leung, Amir Hirsa, Juan Lopez Numerous scientific, industrial, and consumer applications continue to emerge for surface-active monomolecular films (surfactant monolayers), yet no reliable predictive model exists for these monolayer systems. Most commonly, flow of a monolayer-covered interface is assumed to be Newtonian and is treated with the Boussinesq-Scriven surface model coupled to the Navier-Stokes equations for the bulk. The model usually breaks down when applied to flows where surface dilatational viscosity is significant. This may be attributed to the presence of coexisting phase domains in the monolayer, which are not explicitly considered in the surface model. In the present study, the flow of a monolayer-covered interface in a periodically driven elongated cavity is measured using two methods. ``Microscopic'' velocity measurements are obtained by tracking individual coexisting phase domains using Brewster angle microscope (BAM) images. ``Macroscopic'' velocity measurements are obtained by performing DPIV cross-correlation on the BAM images which include numerous domains. Although both of these methods provide microscopically precise measurements of the velocity, only the DPIV on BAM method provides statistically relevant data. The ``macroscopic'' results from DPIV on BAM provide results consistent with the predictions from the Boussinesq-Scriven model. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JP.00004: Marangoni convection in ethanol and water mixtures Jie Zhang, Robert Behringer, Alexander Oron Marangoni convection has been studied in liquid thin films using a binary mixture of ethanol and water. The films are open to the ambient room air during all experimental runs. The evaporation speed of ethanol is much faster than water under the usual conditions. Different patterns, including novel, mobile, circular-shaped patterns, were observed at the different initial weight fraction of ethanol. Pattern sizes grow substantially with the increment of the ethanol concentration c. The starting patterns are rather small at c=0.1, with randomly distributed dots. When c increases from c=0.15 to c=0.2, circular-shaped patterns gradually appear, covering the whole system. As c further increases, these circular patterns combine to form bigger structures. At c=0.35, a large-scale mean flow was generated. For a fixed concentration, patterns evolve due to the evaporation of both the solvent and the solute. We contrast the present system to what occurs in binary mixture of NaCl and water, where only single component, water, evaporates (Jie Zhang et al. PRE 76, 016306 (2007)). [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JP.00005: Marangoni-driven Spreading along Liquid-Liquid Interfaces Steffen Berg Marangoni-driven spreading has been studied extensively at gas-liquid interfaces but so far the spreading kinetics along the interface between immiscible liquids has not been investigated systematically. For a demonstration of the mechanism the spreading kinetics of aqueous surfactant solutions along the interface between water and decane is measured experimentally using laser shadowgraphy. The leading edge follows a power law where the radius $r$ as function of time $t$ scales as $r(t) \propto t^{3/4}$. Ater extending the existing model for spreading at air-liquid interfaces by taking the viscous dissipation in both fluid layers below and above the interface into account, quantitative agreement between experimental data and the model is obtained. Marangoni-driven spreading along an interface is a fast transport mechanism. The velocity of the leading edge is in the range of group velocities of capillary waves. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JP.00006: The deformation of an elastic ring by surface forces Tom Mullin We present the results of experimental investigations into instabilities in an elastic ring floating on a fluid. An initially circular ring sits on the surface of a fluid filled cone and is compressed as the liquid is drained away. The compressive forces give rise to buckling instabilities in the ring and these can be related to results for the compression of solid rings within cylinders. The effects of outer boundary shapes have been explored where anisotropic forcing has produced interesting cases. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JP.00007: Air entrainment by a liquid jet Etienne Reyssat, David Quere We describe experimental work on air entrainment by a liquid jet impacting a bath of the same viscous liquid. We show that the entrainment velocity is shifted due to the widening out of the jet before its impact. The apparent entrainment threshold is determined quantitatively. The thickness of the entrained air film is measured and described by a Landau and Levich model. We also study the impact of a fibre coated with oil on a bath of the same oil. This modified setup allows a simpler analysis of the results and is a good experimental support to the jet problem. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JP.00008: Formation of water bells Christophe Ybert, Christophe Clanet, Lyderic Bocquet, Cyril Duez We study experimentally the situation that consist in a liquid jet impacting normally onto a fixed solid disk. Depending on the experimental conditions, the thin liquid film that spreads onto the solid surface can either pour along the surface, or detach form the disk and form a so-called water bell. The dynamics and the stability of such bells as a function of the hydrodynamic parameters such as the jet and disk diameters or the jet velocity, have already been the object of detailed characterization [1]. This experiment of bell formation appears as the symmetric situation compared to that of a solid body impacting a quiescent liquid. In the latter case, it was recently shown [2] that despite large Re and We numbers, the solid surface characteristics were dramatically influencing the impact scenario. In the present study, we consequently revisit this problem of water bell formation by systematically varying the solid surface characteristics (roughness, surface properties, etc.). It is shown here again that surface parameters strongly influence the domain of bell existence. Our measurements are rationalized by a subtle balance between inertia versus capillary forces and wetting contributions on the liquid film in the ejection region. [1] C. Clanet, {\it J. Fluid Mech.}, 430, 111-147 (2001) [2] C. Duez {\it et al.}, {\it Nature Physics}, 3, 180-183 (2007) [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JP.00009: Micro-step Control of the Position of Oil Droplets Floating in an Aqueous Solution Kyuyong Lee, Chaeyeon Song, Jong Kyun Moon, Hyuk Kyu Pak We experimentally investigated the controlled movement of micro-liter oil droplets on the surface of an aqueous solution induced by laser heating. When heated by laser on one side for a short time, the droplet moves toward the laser with a finite micron-scale step, which can be determined by the intensity of the laser and the duration of heating. Thus, the position of droplets can be precisely controlled by applying a series of laser heating. We show that this stepwise movement of the droplet is due to the contact line advance caused by laser heating and the following capillary flow induced by the imbalance in the Laplace pressure. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JP.00010: Optical Manipulation of Oil Droplets Floating in an Aqueous Solution Chaeyeon Song, Jong Kyun Moon, Kyuyong Lee, Hyuk Kyu Pak We experimentally investigated the motion of micro-liter oil droplets on the surface of an aqueous solution induced by laser heating. The droplets exhibit two types of motion, directed movement parallel to the laser beam and periodic pumping -- oscillation of the contact line. The directed movement of the droplet can be switched between forward and backward by changing the optical path of the laser through the droplet. We can also control the onset of the pumping and its frequency by changing the laser intensity and the heating point on the droplet. We show that the mechanism of both types of motion can be explained in terms of the contact angle adjustment, the resulting force imbalance at the contact line of the droplet, and the convective flow in the droplet. [Preview Abstract] |
Monday, November 19, 2007 5:45PM - 5:58PM |
JP.00011: Wicking within forests of micro-pillars Mathilde Reyssat, Chieko Ishino, Etienne Reyssat, Ko Okumura, David Quere We describe how a wetting liquid brought into contact with a forest of micro-pillars impregnates this forest. Both the driving and the viscous forces depend on the parameters of the texture (radius b and height h of the pillars, pitch p of the network) and it is found that two different limits characterize the dynamics of wicking. For small posts (h $<$ p), the film progresses all the faster since the posts are high, allowing a simple control of this dynamics. For tall pillars (h $>$ p), the speed of impregnation becomes independent of the pillar height, and becomes mainly fixed by the radius of the posts. [Preview Abstract] |
Monday, November 19, 2007 5:58PM - 6:11PM |
JP.00012: Numerical analysis of capillary flow by optimal control Masahiko Sugiura, Keiji Kawachi Inspired by a water strider, the development of micro machines walking on water is in progress. In order to establish design principles for the micro machines walking on water, it is important to know what cross-sectional shape of the supporting legs is efficient to locomote. In this paper, cross-sectional shape of a bar generating large propulsive force in a steady motion is discussed. It is possible to measure the drag acting on various kinds of cross-sectional shapes in experiment, but it is efficient if we can estimate drag by numerical method. This paper focuses on a new numerical method for estimating drag acting on a partially submerged object. The method eliminates computational cost by using optimal control and enables accurate modeling of two-dimensional fluid flows driven by surface tension. [Preview Abstract] |
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