Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G36: Bubbles: Surfactants and Foams |
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Chair: Peter Stewart, University of Glasgow Room: Ballroom C |
Monday, November 23, 2015 8:00AM - 8:13AM |
G36.00001: Microstructural effects in foam fracture Peter Stewart, Stephen Davis, Sascha Hilgenfeldt We examine the fracture of a quasi two-dimensional aqueous foam under an applied driving pressure, using a network modelling approach developed for metallic foams by Stewart \& Davis ({\it J.~Rheol.}, vol.~56, 2012, p.~543). In agreement with experiments, we observe two distinct mechanisms of failure analogous to those observed in a crystalline solid: a slow ductile mode when the driving pressure is applied slowly, where the void propagates as bubbles interchange neighbours through the T1 process, and a rapid brittle mode for faster application of pressures, where the void advances by successive rupture of liquid films driven by Rayleigh--Taylor instability. The simulations allow detailed insight into the mechanics of the fracturing medium and the role of its microstructure. In particular, we examine the stress distribution around the crack tip and investigate how brittle fracture localizes into a single line of breakages. We also confirm that pre-existing microstructural defects can alter the course of fracture. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G36.00002: Patterns, Instabilities, Colors, and Flows in Vertical Foam Films Subinuer Yilixiati, Ewelina Wojcik, Yiran Zhang, Krupa Shah, Vivek Sharma Understanding and controlling the drainage kinetics of thin films is an important problem that underlies the stability, lifetime and rheology of foams and emulsions. We follow the drainage kinetics of vertical foam films using imaging and color science. Interference between light reflected from two surfactant-laden surfaces that are 100 nm - 10 micron apart leads to thickness-dependent iridescent colors in the visible region. Below 50 nm the thin films appear as black. In this study, we utilize the thin film interference colors as markers for identifying patterns, instabilities and flows within vertical foam films. We study the emergence of thickness fluctuations near the borders (i.e. marginal regeneration) and within thinning films. Finally, we elucidate how buoyancy, capillarity, convection and gravity-driven instabilities and flows, are affected by the choice and concentration of constituents. We find fascinating examples of two-dimensional hydrodynamics and unexplained, if not unprecedented, drainage kinetics. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G36.00003: Domain growth kinetics in stratifying foam films Yiran Zhang, Vivek Sharma Baking bread, brewing cappuccino, pouring beer, washing dishes, shaving, shampooing, whipping eggs and blowing bubbles all involve creation of aqueous foam films. Typical foam films consist of two surfactant-laden surfaces that are $\sim$ 5 nm -- 10 micron apart. Sandwiched between these interfacial layers is a fluid that drains primarily under the influence of viscous and interfacial forces, including disjoining pressure. Interestingly, a layered ordering of micelles inside the foam films (thickness \textless 100 nm) leads to a stepwise thinning phenomena called stratification, which results in a thickness-dependent variation in reflected light intensity, visualized as progressively darker shades of gray. Thinner, darker domains spontaneously grow within foam films. We show that the domain expansion dynamics exhibit two distinct growth regimes with characteristic scaling laws. Though several studies have focused on the expansion dynamics of isolated domains that exhibit a diffusion-like scaling, the change in expansion kinetics observed after domains contact with the Plateau border has not been reported and analyzed before. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G36.00004: Gravitational Drainage of Foam: Planar Films, Stability and Foamability Soumyadip Sett, Rakesh Sahu, Alexander Yarin Gravitational drainage from thin plane vertical surfactant films was studied experimentally by using microinterferometry. Anionic surfactant Sodium Dodecyl Sulfate (SDS), cationic surfactant Dodecyltrimethylammounium Bromide (DTAB), nonionic surfactant Tetraethylene Glycol Monooctyl Ether (C8E4) and nonionic superspreader trisiloxane SILWET L-77 were used. The experimenta results were used to measure the Gibbs surface elasticities of these surfactants, as well as the disjoining pressure of the superspreader. The interpretation of the experimental results was based on the theoretical model developed in the present work. Foamability and foam stability of foams generated from these surfactant solutions were studied experimentally in a settler column. Solutions and foams of SDS and the superspreader mixtures were also studied, and the resulting mechanism of drainage deceleration uncovered. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G36.00005: Coupling thermocapillary and solutocapillary stress in 2D micro-foam drainage Marie-Caroline Jullien, Vincent Miralles, Emmanuelle Rio, Isabelle Cantat The foam drainage dynamics is known to be strongly affected by the nature of the surfactants stabilising the liquid/gas interface. In the present work, we consider a 2D microfoam stabilized by both soluble (sodium dodecylsulfate) and insoluble (dodecanol) surfactants. The drainage dynamics is driven by a thermocapillary Marangoni stress at the liquid/gas interface [V. Miralles et al., Phys. Rev. Lett., 2014] and the presence of dodecanol at the interface induces a solutocapillary stress acting against the applied thermocapillary stress, hence slowing down the drainage dynamics. We define a dimensionless permeability of the 2D foam in order to get insight into the relative contributions of the two surface stresses at play. We propose different surfactant transport scenarios. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G36.00006: Disorder growth in a monodisperse foam in microfluidics Nicolas Taccoen, Benjamin Dollet, Charles Baroud Monodisperse foam destabilisation is a complex problem and concerns various applications. For instance, the geometric structure of a foamed gel or concrete must be preserved until the matrix sets. Here we study experimentally this problem by observing, in microfluidics , the evolution of a monolayer of $\sim$30'000 spherical bubbles (radius~0.1mm). We are able to individually track their positions and radii during 20h. We observe a transition from a highly ordered crystalline state (polydispersity=3\%) to a completely disorder amorphous state (polydispersity=30\%). This final state follows the scaling laws predicted by the classical LSW theory. To describe the transition, we define a geometric criterion that classifies the bubbles in disordered or ordered population. We observe the nucleation and growth of disorder zones, while large ordered zones remain. We show that the destabilisation of the foam is not a homogeneous process, but is the combination of two effects: (i) the quick desabilisation inside disordered zones, (ii) the growth in size of these zones, at the expense of the monodisperse ordered zones. Finally, we measure the volume variation rate of each bubble and show that while most of the gas transfer occurs in disordered zones, activity exists in ordered zones. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G36.00007: Modeling Coarsening Induced Foam Drainage Using the Arbitrary Lagrangian Eulerian Method Andrew Brandon, Ramagopal Ananth In this presentation, we will explore coarsening induced foam drainage. Coarsening is the process by which a foam’s average bubble size increases over time due to diffusion of dissolved gas. Through bubble surface movement, coarsening induces drainage and these two processes are capable of altering the foam’s properties. Current models have explained some aspects of these coupled processes, but there remain questions that these foam-scale models cannot answer. To address some of these questions, we have created a bubble-scale Arbitrary Lagrangian Eulerian model of an idealized, coarsening foam. Drainage is captured by solving the Navier-Stokes equations over the foam’s liquid domain and bubble interface movement is described by equations derived to govern the exchange of gas between bubbles. With this model, we have studied the impact that assuming constant film thicknesses (the distance between bubbles) in the coarsening equations can have on drainage. This assumption is typical in current foam-scale models. In this presentation, we will show that allowing the film thicknesses to vary results in a better representation of coarsening induced drainage. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G36.00008: Flow of foams in two-dimensional disordered porous media Benjamin Dollet, Baudouin Geraud, Sian A. Jones, Yves Meheust, Isabelle Cantat Liquid foams are a yield stress fluid with elastic properties. When a foam flow is confined by solid walls, viscous dissipation arises from the contact zones between soap films and walls, giving very peculiar friction laws. In particular, foams potentially invade narrow pores much more efficiently than Newtonian fluids, which is of great importance for enhanced oil recovery. To quantify this effect, we study experimentally flows of foam in a model two-dimensional porous medium, consisting of an assembly of circular obstacles placed randomly in a Hele-Shaw cell, and use image analysis to quantify foam flow at the local scale. We show that bubbles split as they flow through the porous medium, by a mechanism of film pinching during contact with an obstacle, yielding two daughter bubbles per split bubble. We quantify the evolution of the bubble size distribution as a function of the distance along the porous medium, the splitting probability as a function of bubble size, and the probability distribution function of the daughter bubbles. We propose an evolution equation to model this splitting phenomenon and compare it successfully to the experiments, showing how at long distance, the porous medium itself dictates the size distribution of the foam. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G36.00009: Foam-Driven Fractures of an Elastic Matrix Ching-Yao Lai, Sam Smiddy, Howard Stone We report an experimental study of foam-driven fractures in an elastic matrix. When a pressurized foam is constantly injected into a gelatin matrix with a constant flow rate, the foam generates a disc-like fracture which is commonly observed in liquid-driven fractures. Compare to liquid-driven fractures, foam-driven fractures grow faster with time. We investigate how the rheological behaviour of foams affects the fracture characteristics by varying the air volume fraction of the foam, the types and concentration of surfactants in the foam. Foam-fracturing reduces the environmental costs of hydraulic fracturing, which inspires this laboratory study. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G36.00010: Enhanced dissolution of particle-stabilized bubbles by cooling Vincent Poulichet, Valeria Garbin Foams and emulsions that are durable and stable under varying environmental conditions (e.g. temperature, humidity) are central in the food and personal care industry. Small bubbles ($< 100\ \mu$m) need to be stabilized against dissolution even in a gas-saturated liquid, because the Laplace pressure drives diffusion across the curved gas-liquid interface. Solid particles adsorbed at the interface of microbubbles have been shown to prevent coalescence and also arrest bubble dissolution. We studied the effect of changes in temperature on the lifetime of particle-stabilized microbubbles. We report a mechanism of destabilization beyond dissolution arrest, driven by the cooling of the external liquid. We show that the dominant mechanism of destabilization is the increase in solubility of the gas in the liquid, leading to a condition of undersaturation, which drives gas diffusion. Control experiments show that indeed, at constant temperature and pressure, undersaturation alone is sufficient to cause particle-stabilized bubbles to dissolve. [Preview Abstract] |
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