Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session A17: Surface Tension I |
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Chair: Kathleen Stebe, University of Pennsylvania Room: 320 |
Sunday, November 20, 2011 8:00AM - 8:13AM |
A17.00001: Numerical simulation of drop dynamics with soluble surfactant Michael Booty, Michael Siegel, Qiming Wang We use a hybrid numerical method that combines the boundary integral method with an asymptotic reduction of surfactant solubility effects neighboring a drop interface in the large bulk Peclet number limit. This provides an accurate and efficient means of simulating drop dynamics with soluble surfactant. The influence of surfactant solubility on steady-state solution branches for a drop that is stretched in an axisymmetric extensional flow is presented together with the influence of various parameters, including the capillary and Biot numbers, on unsteady drop evolution and tipstreaming. [Preview Abstract] |
Sunday, November 20, 2011 8:13AM - 8:26AM |
A17.00002: Dispersion of Particles on Fluid-Liquid Interfaces B. Dalal, S. Gurupatham, M. Hossain, I. Fischer, P. Singh, D. Joseph This talk is concerned with the dispersion of particles on the fluid-liquid interface. In our previous studies we have shown that when small particles, e.g., flour, pollen, etc., come in contact with an air-liquid interface, they disperse in a manner that appears explosive. This is due to the fact that the capillary force pulls particles into the interface causing them to accelerate to a relatively-large velocity. The motion of particles in the direction normal to the interface is inertia dominated, and so they oscillate vertically about the equilibrium position before coming to a stop under viscous drag. This causes a radially-outward lateral flow on the interface that causes nearby particles to move away. In experiments the strength of the lateral flow was measured using tracer particles that were placed on the interface for this purpose. The dispersion on a liquid-liquid interface was relatively weaker than on an air-liquid interface, and occurred over a longer period of time. This partly was a consequence of the fact that particles became separated while sedimenting through the upper liquid and reached the interface over a time interval that lasted for several seconds. The rate of dispersion depended on the size of particles, the particle and liquids densities, the viscosities of the liquids involved, and the contact angle. [Preview Abstract] |
Sunday, November 20, 2011 8:26AM - 8:39AM |
A17.00003: Drops on elastic tracks Camille Duprat, Suzie Protiere, Alexander Beebe, Howard Stone Fibrous media are ubiquitous functional materials, which often consist of flexible high aspect ratio fibers that can easily deform under capillary forces with many industrial and ecological consequences. We study the influence of a mist of droplets on an elastic array of fibers by considering a finite volume drop on a pair of two flexible fibers, clamped at one end and free to deflect at the other. The elastocapillary deformation of the fibers leads to the spontaneous motion of the drop toward the free ends. The drop either remains compact with minimal spreading or spreads into a long liquid column that coalesces the fibers. We find that there is a critical volume of liquid, hence a critical drop size, above which this coalescence does not occur, and we identify another drop size which maximizes spreading, thus liquid capture. Experimental results and mathematical models will be presented and compared. These ideas are applicable to a wide range of fibrous materials, as we illustrate with quantitative examples for feathers, beetle tarsi, sprays and microfabricated systems. [Preview Abstract] |
Sunday, November 20, 2011 8:39AM - 8:52AM |
A17.00004: Streaming flows and propulsion by vertically oscillating free surfaces Junqi Yuan, Sung Kwon Cho Vertical oscillations of the free surface generated by AC electrowetting can be used to propel mini-scale floating objects. In this presentation, detailed mechanisms in propulsion are discussed. Flow visualization shows that the vertical oscillations of free surface generate a quasi-steady streaming flow. The overall flow pattern is that the fluid is tangentially drawn from the sides of the electrode and is ejected normal to the electrode. It seems that this streaming flow is responsible for pushing the electrode in an opposite direction to the normal flow. Additionally, the effects of amplitude and frequency of vertical oscillation on the streaming flow pattern and propulsion efficiency are also presented. Finally, it is discussed that the depth of free surface is another key parameter that affects propulsion strength. [Preview Abstract] |
Sunday, November 20, 2011 8:52AM - 9:05AM |
A17.00005: Fabrication of Superhydrophobic Fiber Coatings by DC-Biased AC-Electrospinning M. Gad-el-Hak, F.O. Ochanda, M.A. Samaha, H. Vahedi Tafreshi, G.C. Tepper Mesh-like fiber mats of polystyrene (PS) were deposited using DC-biased AC-electrospinning. Superhydrophobic surfaces with water contact angles greater than 150$^{\circ}$ and gas fraction values of up to 97\% were obtained. A Rheological study was conducted on these fiber surfaces and showed a decrease in shear stress when compared with a noncoated surface (no slip), making them excellent candidates for applications requiring the reduction of skin-friction drag in submerged surfaces. We have also shown that addition of a second, low-surface energy polymer to a solution of PS can be used to control the fiber internal porosity depending on the concentration of the second polymer. Contact-angle measurements on mats consisting of porous and nonporous fibers have been used to evaluate the role of the larger spaces between the fibers and the pores on individual fibers on superhydrophobicity. [Preview Abstract] |
Sunday, November 20, 2011 9:05AM - 9:18AM |
A17.00006: Drop Impingement on Highly Wetting Micro/Nano Porous Surfaces Cullen Buie, Youngsoo Joung Recently, we developed a novel fabrication method using a combination of electrophoretic deposition (EPD) and break down anodization (BDA) to achieve highly wetting nanoporous surfaces with microscale features. In this study we investigate droplet impingement behavior on these surfaces as a function of impact velocity, droplet size, and liquid properties. We observe impingement modes we denote as ``necking'' (droplet breaks before full penetration in the porous surface), ``spreading'' (continuous wicking into the porous surface), and ``jetting'' (jets of liquid emanate from the edges of the wicking liquid). To predict the droplet impingement modes, we've developed a non-dimensional parameter that is a function of droplet velocity, dynamic viscosity, effective pore radius and contact angle. The novel dimensionless parameter successfully predicts drop impingement modes across multiple fluids. Results of this study will inform the design of spray impingement cooling systems for electronics applications where the ``spreading'' mode is preferred. [Preview Abstract] |
Sunday, November 20, 2011 9:18AM - 9:31AM |
A17.00007: Armored droplets from particle rafts and an application to environmental remediation Howard A. Stone, Manouk Abkarian, Suzie Protiere, Jeff Aristoff Applications such as handling hazardous materials or containing chemical reactions involve encapsulating one fluid phase by a shell. In addition to surfactants, a variety of physicochemical approaches have been studied where nano- and/or micron-size particles organize at fluid-fluid interfaces to form some sort of shell. Gravity typically plays an insignificant role compared to surface forces in establishing such ``armored interfaces.'' More generally, however, it is well known that capillary and gravitational forces cause particles trapped at interfaces to self-assemble and organize into raft-like structures. We describe gravity-driven instabilities of particle rafts and show how the ``interfacial granular dynamics'' lead to encapsulation strategies involving stable particle-armored droplets. Experimental results are compared with mathematical models of the composite objects in order to establish a quantitative description of our observations. The application of these ideas to environmental remediation will be described. [Preview Abstract] |
Sunday, November 20, 2011 9:31AM - 9:44AM |
A17.00008: Capillary attraction between floating cylinders Harish N. Dixit, George M. (Bud) Homsy Capillary attraction between floating particles is a phenomenon of everyday experience and causes the particles at fluid interfaces to agglomerate. This is sometimes called the ``Cheerios effect.'' The calculation of the force of attraction for even simple particle shapes is enormously difficult due to the presence of nonlinearity in the equations governing the interface shape. Most of the earlier approaches to obtained analytical expression for the force of attraction rely on the ``superposition principle'' by linearizing the Young-Laplace equation. We use a systematic perturbation technique in the limit of small Bond number. We present asymptotic solutions in a static configuration obtained for two problems: (i) attraction between two isolated cylinders, and (ii) attraction for an infinite array of cylinders. It is also found that the background curvature of an interface modifies the force of attraction. These forces are important in many technological applications: one such example is the problem of dip-coating of plates which will be discussed during the meeting. [Preview Abstract] |
Sunday, November 20, 2011 9:44AM - 9:57AM |
A17.00009: Coalescence-induced droplet actuation Mathieu Sellier, Claude Verdier, Volker Nock This work investigates a little explored driving mechanism to actuate droplets: the surface tension gradient which arises during the coalescence of two droplets of liquid having different compositions and therefore surface tensions. The resulting surface tension gradient gives rise to a Marangoni flow which, if sufficiently large, can displace the droplet. In order to understand, the flow dynamics arising during the coalescence of droplets of different fluids, a model has been developed in the lubrication framework. The numerical results confirm the existence of a self-propulsion window which depends on two dimensionless groups representing competing effects during the coalescence: the surface tension contrast between the droplets which promotes actuation and species diffusion which tends to make the mixture uniform thereby anihilating Marangoni flow and droplet motion. In parallel, experiments have been conducted to confirm this self-propulsion behaviour. The experiment consists in depositing a droplet of distilled water on a ``hydrophilic highway.'' This stripe was obtained by plasma-treating a piece of PDMS shielded in some parts by glass coverslips. This surface functionalization was found to be the most convenient way to control the coalescence. When a droplet of ethanol is deposited near the ``water slug,'' coalescence occurs and a rapid motion of the resulting mixture is observed. [Preview Abstract] |
Sunday, November 20, 2011 9:57AM - 10:10AM |
A17.00010: Immiscible surfactant droplets on thin liquid films: Spreading dynamics, subphase expulsion, oscillatory instabilities and the effect of spatial confinement David Sinz, Myroslava Hanyak, Anton Darhuber After deposition of immiscible, surface-active liquids on thin liquid films of higher surface tension, Marangoni stresses thin the liquid film around the surfactant droplet and induce a flow directed away from the surfactant source. We present a combined experimental and numerical study elucidating a variety of aspects of this phenomenon. In a radially symmetric configuration we investigated an oscillatory instability, caused by temporary trapping and subsequent release of subphase liquid from underneath the surfactant droplet. We provide evidence that this expulsion has a pronounced effect on the spreading dynamics. Using chemical surface patterning we study the effect of spatial confinement of the subphase liquid on the spreading dynamics. The lateral confinement induces non-uniform height- and surface velocity profiles, which manifest themselves in a pronounced transition in the time evolution of the subphase morphology. With respect to both the spreading rates as well as the evolving morphology, excellent agreement between experimental and simulation results has been achieved. [Preview Abstract] |
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