Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session GD: Foams |
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Chair: Michael Dennin, University of California, Irvine Room: Hilton Chicago Continental A |
Monday, November 21, 2005 10:34AM - 10:47AM |
GD.00001: Speckle Visibility Spectroscopy and Bubble Rearrangements Alex Gittings, Douglas Durian We use Speckle Visibility Spectroscopy (SVS) to study bubble rearrangements in aqueous foams. SVS is a new dynamic light scattering technique based upon the visibility of a speckle pattern for a given exposure time [1]. Unlike more traditional dynamic light scattering, such as diffusing wave spectroscopy, the dynamics are determined from an ensemble average over camera pixels. This allows resolution of motion that changes systematically and rapidly with time. Our foam sample is contained within a thick glass cell, one face of which is entirely covered with absorbing black tape. Laser light is both introduced and collected at a 1-mm diameter hole punched in the tape. We use a 50 kHz line scan camera and image several speckles per pixel. Probability distributions are determined for the bubble velocity, duration, and time between rearrangements events for several foam ages.\newline \newline [1] Speckle Visibility Spectroscopy and Variable Granular Fluidization P. K. Dixon and D. J. Durian, Phys. Rev. Lett. 90, 184302 (2003). [Preview Abstract] |
Monday, November 21, 2005 10:47AM - 11:00AM |
GD.00002: Transport of Gas in Steady State Aqueous Foam Klebert Feitosa, Douglas J. Durian An experiment is performed to investigate the transport of gas in a column of aqueous foam. The foam is maintained in steady state by a constant flux of gas at the bottom. The bubble velocity, liquid-fraction and bubble-size vertical profiles are measured in the sample. The results show that in steady state the bubble velocity is constant, the liquid-fraction profile is set predominantly by the viscous drag, and the coarsening rate depends on the inverse of the square root of liquid fraction. These findings provide a simple description of steady state foams via the drainage and coarsening equations. [Preview Abstract] |
Monday, November 21, 2005 11:00AM - 11:13AM |
GD.00003: Local Topological Dynamics in Sheared 2D Foams Kapilanjan Krishan, Yuhong Wang, Michael Dennin We describe our experiments on two dimensinal foams subject to parallel shear. The system provides insights into shear localization in glassy systems. The dynamics of the system are described through statistics on the local topological rearrangements through neighbour rearrangements as well as average velocity profiles. The statistics indicate qualitative differences in the dynamics of the flow associated with boundary conditions affecting the wettability of the foam. In particular, we correlate the distribution of T1-events with localized regions of plug-flow. [Preview Abstract] |
Monday, November 21, 2005 11:13AM - 11:26AM |
GD.00004: Large-scale foam flows: discrete effects and limits of a continuum approach. Igor Veretennikov, Marius Asipauskas, James Glazier One might expect a continuum approach to apply to a flowing foam if all spatial scales of the flow are much larger than the size of the individual bubbles. Our experiments on two-dimensional foams flowing through constrictions and around obstacles show that this assumption often fails. Any high-stress regions in the flow cause structural changes (in particular, topological rearrangements (T1 processes)) at preferred locations, which can induce large amplitude jumps in average foam velocity and/or streamline splitting. The resulting flows differ qualitatively from continuum flows ({\it e.g.} flow velocity may be maximal at a classical stagnation point. Because topological changes always occur at the bubble scale, a continuum description cannot hold. We explain these phenomena qualitatively. [Preview Abstract] |
Monday, November 21, 2005 11:26AM - 11:39AM |
GD.00005: Phase-field simulation of gas bubble growth and flow in a Hele-Shaw cell Ying Sun, Christoph Beckermann A diffuse interface model has been developed for gas bubble growth and dynamics in a supersaturated liquid. The liquid becomes supersaturated in the gas species because of a drop in the pressure or temperature. The bubbles grow by gas diffusion in the liquid towards the bubble interfaces. During bubble growth, flows are induced by the large density contrast between the phases. The bubbles coarsen due to surface tension effects. The process widely exists in biological systems, materials processing, oil recovery, and other applications. The flows in the gas and liquid phases are solved using a diffuse interface model for two-phase flows with surface tension, phase change, and density and viscosity differences between the phases. This diffuse-interface model for flow is coupled with a phase-field equation for calculating the interface motion, and a species conservation equation for the gas transport. The model is validated for a single bubble growing inside a semi-infinite liquid, and convergence of the results with respect to the interface width is demonstrated. Large-scale numerical simulations for multiple bubbles inside a Hele-Shaw cell reveal the presence of complex interface dynamics and flows. The bubble dynamics, including coarsening and coalescence, are investigated as a function of the initial gas concentration, surface tension, and the density and viscosity contrasts between the phases. [Preview Abstract] |
Monday, November 21, 2005 11:39AM - 11:52AM |
GD.00006: Rigidity percolation in particle laden foams Sylvie Cohen-Addad, Reinhard Hohler, Marcel Krzan, Marijo Marinic, Benjamin Herzhaft Aqueous foams are concentrated dispersions of gas bubbles in a soapy solution. These complex fluids exhibit solid-like or liquid-like mechanical behaviors depending on the applied shear. We study how their viscoelasticity and their yielding are modified when non colloidal solid particles are dispersed in the foam. We show that even small amounts of particles can enhance the viscoelastic shear modulus by more than an order of magnitude. The scaling of the elasticity enhancement with particle concentration qualitatively agrees with that expected for percolation on superelastic networks constituted of strong bonds randomly dispersed in a soft matrix. A plausible candidate for these bonds might be the capillary bridges between particles. Moreover, we show that the yield stress of particle laden foams increases only linearly with the particle concentration, suggesting that percolating bond chains are stiff but rupture easily. The effects of particle to bubble size ratio as well as particle wettability and shape are also investigated. [Preview Abstract] |
Monday, November 21, 2005 11:52AM - 12:05PM |
GD.00007: Foams for Microfluidics Jan-Paul Raven, Philippe Marmottant, Fran\c{c}ois Graner We present an experimental investigation of the assembly of microbubbles into a 2D foam and its flow in microchannels. Using a flow focusing method, we can produce a foam \textit{in situ} on a microfluidic chip for a large range of liquid fractions. We study its flow dissipation along a channel, and the effect of constrictions. Microscopic imaging allows monitoring the transition from separated bubbles into the desired foam in which bubbles are closely packed. The foam flowrate depends non-linearly on the applied pressure, displaying a threshold pressure due to capillarity. The measurements are made in a channel with a height of 250 $\mu m$, resulting in bubbles whose height to diameter aspect ratio ranges between 0.3 and 1. We also produce an ultraflat foam (reducing the channel height to about 8 $\mu m$) with a bubble aspect ratio down to 0.02; we observe a marked change in bubble shape during the flow. The control of microfoam flows provides possible applications like transporting amphiphilic molecules on interfaces or the individual handling of gas pockets. [Preview Abstract] |
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