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 GW: Surface Tension I |
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Chair: Ho-Young Kim, Seoul National University Room: Hyatt Regency Long Beach Regency C |
Monday, November 22, 2010 8:00AM - 8:13AM |
GW.00001: Capillary wrinkling of elastic membranes Dominic Vella, Mokhtar Adda-Bedia, Enrique Cerda We present a physically-based model for the deformation of a floating elastic membrane caused by the presence of a liquid drop. Starting from the equations of membrane theory modified to account for the three surface tensions in the problem, we show that the presence of a liquid drop causes an azimuthal compression over a finite region. This explains the origin of the wrinkling of such membranes observed recently [J. Huang et al., Science 317, 650 2007] and suggests a single parameter that determines the extent of the wrinkled region. While experimental data supports the importance of this single parameter, our theory under-predicts the extent of the wrinkled region observed experimentally. We suggest that this discrepancy is likely to be due to the wrinkling observed here being far from threshold and discuss other, related, geometries. [Preview Abstract] |
Monday, November 22, 2010 8:13AM - 8:26AM |
GW.00002: A Computational Study of Surfactant Effects in the Bretherton Problem Metin Muradoglu, Ufuk Olgac, Gokalp Gursel The finite-difference/front-tracking method developed by Muradoglu and Tryggvason (2008) is used to study the motion and deformation of a large bubble moving through a capillary tube in the presence of both insoluble and soluble surfactants. Emphasis is placed on the effects of surfactant on the liquid film thickness between the bubble and tube wall. The numerical method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations. A non-linear equation of state is used to relate interfacial surface tension to surfactant concentration at the interface. The method is validated for simple test cases and the computational results are found to be in a good agreement with the analytical solutions. The film thickness is first computed for the clean bubble case and the results are compared with the lubrication theory in the limit of small capillary numbers, i.e., $Ca \ll1$. Finally the method is used to simulate the effects of insoluble and soluble surfactants on the film thickness for a wide range of governing non-dimensional numbers. [Preview Abstract] |
Monday, November 22, 2010 8:26AM - 8:39AM |
GW.00003: Vapor stabilizing surfaces for superhydrophobicity Neelesh Patankar The success of rough substrates designed for superhydrophobicity relies crucially on the presence of air pockets in the roughness grooves. This air is supplied by the surrounding environment. However, if the rough substrates are used in enclosed configurations, such as in fluidic networks, the air pockets may not be sustained in the roughness grooves. In this work a design approach based on sustaining a vapor phase of the liquid in the roughness grooves, instead of relying on the presence of air, is explored. The resulting surfaces, referred to as vapor stabilizing substrates, are deemed to be robust against wetting transition even if no air is present. Applications of this approach include low drag surfaces, nucleate boiling, and dropwise condensation heat transfer, among others. [Preview Abstract] |
Monday, November 22, 2010 8:39AM - 8:52AM |
GW.00004: Dynamic surface tension effects from molecular dynamics simulations Alex Lukyanov, Alexei Likhtman Effects of dynamic surface tension have been studied in a model system using molecular dynamics simulations. The model system has been made of an artificially expanding liquid droplet, with the rate of change of the external surface area being comparable with the gas-liquid interface formation characteristic time, obtained from the estimates of macroscopic theories. The size of the liquid droplet has been chosen to have about 5,000-7,000 identical chain molecules, each having between 10-20 beads, to obtain well developed and separated the bulk and the surface phases. The methodology of surface tension evaluation has been verified against the Laplace Law in a stationary state of the liquid drop. The results of the molecular dynamics simulations will be discussed in comparison with the estimations obtained from macroscopic experiments on dynamic wetting using a sharp interface formation theory for different chain length of molecules and strength of intermolecular interactions. [Preview Abstract] |
Monday, November 22, 2010 8:52AM - 9:05AM |
GW.00005: The Dynamics of Droplets and Holes in Thin Surfactant Films Karen Daniels, Kali Allison, Jonathan Claridge, Rachel Levy, Ellen Peterson, Michael Shearer, Wynn Vonnegut, Jeffrey Wong We perform quantitative measurements of the spreading of an insoluble surfactant on a thin layer of glycerin, starting from either a droplet or hole (anti-droplet) configuration. We make direct measurements of both the radial height profile and the spatial distribution of the fluorescently-tagged surfactant during the spreading process. Remarkably, the surfactant dynamics are quite different for the two cases. We compare these experimental results to numerical solutions using lubrication theory for thin films. [Preview Abstract] |
Monday, November 22, 2010 9:05AM - 9:18AM |
GW.00006: Long term stability of immiscible ferrofluid/water interfaces Bernard Malouin, David Posada, Amir Hirsa Recently we have demonstrated pinned-contact, coupled droplet pairs of aqueous ferrofluids in air that can form electromagnetically-activated capillary switches and oscillators. The great variety of available ferrofluids, however, enables the use of immiscible oil-based ferrofluid droplets in a water environment to obtain the same behavior. Such immersed ferrofluid oscillators exhibit natural frequencies (for 5 mm devices) of about 10 Hz. Here we report on the observation of a gradual \textit{increase} in the resonant frequency of the system in time. Experimental observations suggest that the drift in the natural frequency is a consequence of changes occurring at the ferrofluid/water interface. The interfacial structure of such a complex system (water, oil, surfactant, iron particles) is examined along with its evolution in time, using various microscopy techniques. [Preview Abstract] |
Monday, November 22, 2010 9:18AM - 9:31AM |
GW.00007: Forces to Dislodge Rotating Sessile and Pendant Annular Rivulets P.D. Weidman, C.P. Malhotra In a recent PRL Tadmor, et al (2009) measured the lateral adhesion force on sessile and pendant drops (oil) of equal volume placed at R = 100 cm on a rotating flat surface (treated mica substrate) and found that more force is required to radially displace the pendant drops. This was explained as enhanced chemical interaction between liquid and solid molecules when the drop is pendant compared to sessile. We take the view it is primarily static advancing and receding contact angles that govern the movement of the drops. This is shown by a simple model where the isolated drop is replaced by a thin axisymmetric rivulet. For realistic advancing and receding contact angles of water on anodized aluminum, computations performed show the existence of four distinct regions governing drop movement. The three regions found at small radii give way at R = 2.0 cm to the final fourth region where drop movement depends on the advancing contact angle for both sessile and pendant drops; here the pendant drops require a larger radial force to dislodge, in agreement with the mesurements of Tadmor, et al. Simulations more closely mimicking the experiments of Tadmor, et al will be presented. [Preview Abstract] |
Monday, November 22, 2010 9:31AM - 9:44AM |
GW.00008: Filtration by eyelashes Krishna Vistarakula, Mike Bergin, David Hu Nearly every mammalian and avian eye is rimmed with lashes. We investigate experimentally the ability of lashes to reduce airborne particle deposition in the eye. We hypothesize that there is an optimum eyelash length that maximizes both filtration ability and extent of peripheral vision. This hypothesis is tested using a dual approach. Using preserved heads from 36 species of animals at the American Museum of Natural History, we determine the relationship between eye size and eyelash geometry (length and spacing). We test the filtration efficacy of these geometries by deploying outdoor manikins and measuring particle deposition rate as a function of eyelash length. [Preview Abstract] |
Monday, November 22, 2010 9:44AM - 9:57AM |
GW.00009: The hydrodynamics of ink writing Jung Chul Kim, Ho-Young Kim When one writes on the paper with a pen, the ink spreads on the porous hydrophilic solid surface leaving a trail whose width depends on the pen speed and the physicochemical properties of the ink and of the paper. Here we mathematically describe the spreading profile of the ink, which is modeled to be a liquid from a tube wicking through a rough hydrophilic surface (micropillar arrays). By balancing the capillary forces that drive the liquid flow with the viscous forces exerted by the forest of micropillars, we obtain the rate of ink spreading. Considering the liquid spreading and the pen translation that occur simultaneously, we predict the frontal shape and the final width of a line that is drawn by the pen. We performed experiments using various kinds of liquids and different dimensions of micropillar arrays that are coated with Si-DLC film and treated with oxygen plasma. The theory and experiment are shown to be in excellent agreement. [Preview Abstract] |
Monday, November 22, 2010 9:57AM - 10:10AM |
GW.00010: Marangoni patterns N. Nirmal Thyagu, Evelyn Strombom, Daniel Palumbo, Carlos Caicedo, Troy Shinbrot We study Marangoni patterns that emerge when common food dye is dropped into a dish of shallow water. These patterns consist of tendrils and spots that sharpen over time before eventually fading. We demonstrate that the patterns can be modeled using coupled reaction-diffusion equations, where the ``reaction'' terms appear due to a nonlinear dependence of surface tension on dye concentration. We show using a spatio-temporal metric that these patterns are distinct from previously described Turing patterns. [Preview Abstract] |
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