Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session R6: Microfluidics: Capillary III |
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Chair: Shriram Pillapakkam, Temple University Room: 24B |
Tuesday, November 20, 2012 1:00PM - 1:13PM |
R6.00001: Trapping and release of bubbles from a linear pore Anne Juel, Geoffrey Dawson, Sungyon Lee Multiphase flows of practical interest are characterized by complex vessel geometries ranging from natural porous media to man-made lab-on-a-chip devices. Models based on the over-simplification of the pore geometry often suppress fundamental physical behavior. We study the effect on bubble motion of a sudden streamwise expansion of a square tube. The extent to which a bubble driven by constant flux flow broadens to partially fill the expansion depends on the balance between viscous and surface tension stresses, measured by the capillary number $Ca$. This broadening is accompanied by the slowing and momentary arrest of the bubble as $Ca$ is reduced towards its critical value for trapping. For $Ca< Ca_c$ the pressure drag forces on the quasi-arrested bubble are insufficient to force the bubble out of the expansion so it remains trapped. We examine the conditions for trapping by varying bubble volume, flow rate of the carrier fluid, and length of expanded region, and find that $Ca_c$ depends non-monotonically on the size of the bubble. We verify with experiments and a capillary static model that a bubble is released if the work of the pressure forces over the length of the expansion exceeds the surface energy required for the trapped bubble to reenter the constricted square tube. [Preview Abstract] |
Tuesday, November 20, 2012 1:13PM - 1:26PM |
R6.00002: Direct measurements of air layer profiles under impacting droplets using high-speed color interferometry Roeland van der Veen, Tuan Tran, Detlef Lohse, Chao Sun A drop impacting on a solid surface deforms before the liquid makes contact with the surface. We directly measure the time evolution of the air layer profile under the droplet using high-speed color interferometry, obtaining the air layer thickness before and during the wetting process. Based on the time evolution of the extracted profiles, we measure the velocity of air exiting from the gap between the liquid and the solid, and account for the wetting mechanism and bubble entrapment. We determine the entrained bubble volume for varying impact velocities using the air layer profiles and compare it to theory and numerical simulations. The present work offers a tool to accurately measure the air layer profile and quantitatively study the impact dynamics at a short time scale before impact. [Preview Abstract] |
Tuesday, November 20, 2012 1:26PM - 1:39PM |
R6.00003: Encapsulating Ellipsoids in Drops Michael Norton, Teresa Brugarolas, Jonathan Chou, Haim Bau, Daeyeon Lee Large aspect ratio particles were produced by embedding spherical polystyrene particles within a polymer film and subsequently heating and stretching the film. Particles were released by dissolving the film. Using a flow-focusing device, the elongated particles were partially encapsulated within droplets of fluid A, such as water, surrounded by an immiscible fluid B, such as oil. Drop volumes were controlled by adjusting the flow rates of fluids A and B. The contact angle was adjusted indirectly by varying the amount of surfactant adsorbed to the particle surface. The encapsulation process was visualized with a high-speed video camera. We observed cases ranging from partial to complete encapsulation and examined experimentally and theoretically the shape of the interface between fluid A and fluid B as a function of the drop volume. The numerically predicted position of the pinning line and the shape of the drop were compared to experimentally produced conformations and agreed favorably.~ [Preview Abstract] |
Tuesday, November 20, 2012 1:39PM - 1:52PM |
R6.00004: Experimental study of adsorption of particles at two-fluid interfaces Vadim Linevich, Shriram Pillapakkam, Pushpendra Singh The inertia of a particle plays an important role in its motion in the direction normal to a two-fluid interface, and in determining its adsorption trajectory. Although the importance of inertia diminishes with decreasing particle size, on an air-water interface the inertia continues to be important even when the particle size is as small as few nanometers. We have recently shown that the motion of particles in the direction normal to the interface while being adsorbed gives rise to a secondary lateral flow on the interface that causes newly adsorbed particles to disperse. The goal of this study is to experimentally study the adsorption trajectory of particles at two-fluid interfaces. Specifically, we will analyze the effect of particle size on the adsorption trajectory of a single particle, as it is being trapped at an air-water interface. The diameter of a particle will be varied between $\sim$ 100 - 1000 microns. Our experimental setup consists of a high speed camera with a recording speed up to 3000 frames per second which is connected to a horizontal microscope. Captured high speed videos are analyzed to determine the frequency and the amplitude of oscillation during adsorption. [Preview Abstract] |
Tuesday, November 20, 2012 1:52PM - 2:05PM |
R6.00005: Dissipative particle dynamics simulation of a liquid meniscus confined between atomic force microscope tip and substrate Zhen Li, Chuanjin Lan, Yanbao Ma Liquid meniscus forms between the atomic force microscope (AFM) tip and the substrate under ambient humidity. The liquid meniscus affects the AFM measurements and plays an important role in dip-pen nanolithography. To understand the behaviors of the meniscus, a mesoscopic methodology called dissipative particle dynamics (DPD) is utilized to investigate the liquid meniscus confined between AFM tip and a solid surface. Results show that the structure of the liquid meniscus is highly dependent on the wettability properties of the tip and the substrate as well as the tip-to-surface distance. The area of liquid-solid interface increases as the wetting properties of the tip and substrate change from hydrophilic to hydrophobic, which results in a transition of the meniscus shape from convex to concave. The wetting properties of solid surface affect the process of the liquid meniscus breakup as the tip-to-surface distance increase. This nonlinear process is also affected by the surface tension of the liquid, thermal fluctuation and the speed of tip. [Preview Abstract] |
Tuesday, November 20, 2012 2:05PM - 2:18PM |
R6.00006: Capillary-Driven Flow through Optimal Wick Structures for Heat Pipe Applications Yu-Wei Liu, Marin Sigurdson, Carl Meinhart In this study we investigate surface tension-driven fluid motion through the wick structure of a flat heat pipe. Wick designs in flat heat pipes are typically limited by viscous drag and capillary pressure, and do not transport fluids at sufficient rates to meet high-demand cooling requirements. An analytical model is used to describe flow through wick structure with array of channels. The capillary pressure and viscous drag are obtained by calculating surface energy difference and solving Stokes equations, respectively. To verify the model, we conducted wetting tests on the wick samples with different channel dimensions. The model agrees qualitatively with the experiments, but under predicts the viscous drag, which is due to the meniscus effect at the surface. The concave liquid surface increases the pressure drop, and therefore reduces the effective permeability. To include the meniscus effect, we combined Stokes equations with deformed surface in a full 3D simulation by COMSOL V4.2a (COMSOL, Inc., Stockholm, Se). The Young-Laplace law and the curvature equation are used to determine the deformed surface. [Preview Abstract] |
Tuesday, November 20, 2012 2:18PM - 2:31PM |
R6.00007: Droplets interacting with structured microchannel walls Rielle de Ruiter, Michel Duits, Frieder Mugele The presence of water droplets in oil-filled microfluidic systems influences the effective resistance of the microchannels. We study the effect of droplets interacting with topographically structured walls, for example single defects or a periodic array of grooves. Depending on the specific geometry and flow conditions, the droplet is trapped in or released from the structured region. Using a so-called microfluidic comparator (a microfluidic unit to compare flow rates in two channels), we quantify the effect of the presence of the droplet and its deformation by the flow on the effective resistance of the microchannel. These insights can be used to optimize the release of entrapped oil during enhanced oil recovery. [Preview Abstract] |
Tuesday, November 20, 2012 2:31PM - 2:44PM |
R6.00008: Lattice Boltzmann Simulations of Finite-Sized Particles in Interfaces Kevin Connington, Taehun Lee, Jeff Morris The presence of solid particles can play an important role in many multiphase/multi-component flows. For example, particles in the interface of an emulsion can act to stabilize the drop, preventing breakup. We extend an existing free energy-based multi-component Lattice Boltzmann Method (LBM) to handle the transport of immersed solid particles of a finite size. The multi-component LBM can simulate the property differences encountered in a water-air system while eliminating the unwanted phenomenon of spurious currents at equilibrium. The particles are transported by the fluid according to Newtonian dynamics. The total force on a particle is computed by Momentum Exchange (ME), as in single phase flow. However, we introduce a supplemental term to account for the force of the interface on the particle. We validate the inclusion of this forcing term, and demonstrate the capability of the code by performing simulations of drop impact with immersed solid particles and the rupture of a liquid bridge containing particles. [Preview Abstract] |
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