Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session A23: Bubbles: Surfactants and Foams |
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Chair: Douglas Durian, University of Pennsylvania Room: 605 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A23.00001: Energy balance and the velocity gap in foam fracture Sascha Hilgenfeldt, Peter Stewart It has been shown experimentally and in simulations that a quasi-two-dimensional foam bubble layer, when driven under external stress, can display ruptures of successive films that constitute an analogue to brittle type-I fracture. Simplified discrete and continuum models from first principles of fluid dynamics capture the features of propagation, including the existence of a critical speed below which steady fracture cannot be sustained (velocity gap). In order to understand the magnitude and parameter dependence of the velocity gap, it is advantageous to strike an energy balance in the system and thus connect with continuum dynamical fracture mechanics. The crack dynamics is characteristically altered by system size (because of changes in strain energy concentration) as well as by dissipation effects in the foam. Access to an explicit analytical formalism for the modeling of all physical effects complements the simulation results and reveals general principles governing the occurrence of fracture velocity gaps. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A23.00002: Active Foam: Connecting Structure, Dynamics and Control Laurel Kroo, Matthew Bull, Manu Prakash By inflating and deflating voxels within a polydisperse 2-D air-liquid foam, we demonstrate an experimental system where we perturb soft materials in a radially-symmetric manner. These cyclic perturbations can be coordinated spatially and temporally to encode (''write'') mechanical properties into the material. Using both a short-range mechanism (cascades of neighbor-swapping events) and a long-range mechanism (low-frequency acoustic), we discuss how to achieve a robust and morphable active material. Topics of interest include scaling analyses, noise, and accounting for complexity that arises from polydispersity and initial conditions. The goal of this work is to understand fundamental principles of confluent tissues and develop synthetic analogs. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A23.00003: Microfluidic-based foams: a possible template for photonic structures Ilham Maimouni, Maria Russo, Maryam Morvaridi, Janine Cossy, Patrick Tabeling Through the past decades, foams have taken more and more place into our modern world and have been used in a myriad of applications such as insulation building materials, food industry and photo-catalysis thanks to their interesting structural properties. Recently, 2D foams have been investigated to be self-assembled materials exhibiting interesting photonic properties. In the present study, we aim at exploring the 3D foams case. In this perspective, microfluidic technologies are used to develop 3D, solid, highly monodisperse polymeric foams by packing air bubbles in aqueous solution containing a polymer. The bubbles are produced in a PDMS (Polydimethylsiloxane) microfluidic chip and directly assembled in a microfluidic channel giving birth to highly tunable 3D foam. Indeed, by varying fluid pressures, the foam composition and the polymerization process, we manage to sharply control bubbles production and thereby govern the structural properties of the obtained material: porosity, pores size, connectivity and polydispersity. Electromagnetic simulations are then performed to study wave propagation in such foams, revealing very interesting transmission regimes and opening the way for a new technological application of fluid-based systems. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A23.00004: Evaporation induced spontaneous cyclic dimpling in binary mixtures and its role in bubble stability Vineeth Chandran Suja, Javier Tajuelo, Gerald Fuller Liquid mixtures having components with heterogeneous volatilities, such as lubricants, are known to support stable foams in the absence of surfactants. Recent studies have shown this is a consequence of stabilizing Marangoni flows induced by the differential evaporation of liquid components from the surface of the bubble. Interestingly, unlike surfactant induced Marangoni flows, evaporation induced Marangoni flows periodically regenerate the thickness of bubble walls resulting in a fascinating phenomenon known as `spontaneous cyclic dimpling'. This phenomenon has been studied in the context of emulsion stability resulting from surfactant diffusion, however very little is known about evaporation induced spontaneous cyclic dimpling. Here we study this phenomenon in detail by visualizing the spatio-temporal evolution of the wall thickness of single bubbles in binary mixtures using white light interferometry. Various binary mixtures of silicone oils are tested to understand the mechanics of spontaneous cyclic dimpling and its influence on bubble stability as function of species mole fraction, volatilities and viscosities. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A23.00005: Effects of soluble surfactant and viscoelasticity on pressure-driven turbulent bubbly channel flow Zaheer Ahmed, Daulet Izbassarov, Pedro Costa, Outi Tammisola, Metin Muradoglu Particle-resolved direct numerical simulations are performed to investigate the combined effects of soluble surfactant and viscoelasticity on structure of pressure-driven turbulent bubbly channel flow ($Re_\tau= 180$). Incompressible flow equations are solved fully coupled with FENE-P viscoelastic model and soluble interfacial and bulk surfactant concentration equations. A non-linear equation of state relates surface tension to interfacial surfactant concentration. The method is first validated using benchmark turbulent single- and two-phase flows. Then massively parallel simulations are performed to examine effects of viscoelasticity and surfactant on turbulent bubbly flows. We found that clean bubbles move toward the walls due to inertial lift force, resulting in formation of wall-layers and a significant decrease in the flow rate. An addition of strong enough surfactant alters the direction of lateral migration of bubbles resulting in a nearly uniform bubble distribution across the channel. For the viscoelastic case, shear-thinning effects promote inertial lift, enhancing formation of bubbly wall-layers and consequently strong decrease in the flow rate. Formation of wall-layers is determined by the interplay of viscoelasticity and surfactant, when they act together. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A23.00006: Direct Numerical Simulations of the Three Dimensional Dynamics of Surfactant Laden Bursting Bubbles Damir Juric, Ricardo Constante-Amores, Assen Batchvarov, Lyes Kahouadji, Seungwon Shin, Jalel Chergui, Richard Craster, Omar Matar Bursting bubbles play an important role in both industrial applications and nature with $\sim$ 10$^{18}$-10$^{20}$ bubble bursts per second over the oceans, exchanging chemical components or heat between the ocean and the atmosphere. When a bubble is close to a free surface, it forms a hole which leaves an open unstable cavity that will collapse; the change of the interface curvature leads to the formation of a central jet, which breaks into droplets according to the Plateau$-$Rayleigh instability. The surfactant-free interfacial dynamics are well understood but the surfactant-laden ones are not. Neglecting gravity, the Laplace number is the only remaining control parameter measuring the relative importance of surface tension to viscous forces i.e. $La = \rho\sigma R/\mu^2$, where $\rho$, $\mu$, $\sigma$, and $R$ are the liquid density, viscosity, surface tension, and the initial radius of the droplet, respectively. 3D DNS simulations varying the Peclet number, $Pe=UR/D$, where $U$ and $D$ denote the velocity and diffusion coefficient, respectively, were performed to analyse the fate of the jet. Results regarding the interfacial surfactant concentration distribution, the surface tension gradients, and the importance of Marangoni stresses on the jet formation will be presented. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A23.00007: The effect of surfactant on the tail dynamics of elongated bubbles Omar Matar, Assen Batchvarov, Mirco Magnini, Lyes Kahouadji, Cristian Constante-Amores, Richard Craster Following the classical work by Bretherton and Taylor, the propagation of elongated gas bubbles in micro-channels has received a great deal of attention in the literature due to its relevance to a wide range of applications. Recent work by Magnini {\it et al}. Phys. Rev. Fluids, 023601 (2018) and Moran {\it et al.} (Bulletin of the American Physical Society, vol. 63, D09.00002) has also examined these flows (with concurrent liquid flow) with significant buoyancy and inertial effects in both vertical and horizontal configurations. The present work focuses on bubble tail undulations in the presence of surfactants whose effects merit attention. We carry out three-dimensional direct numerical simulations of the flow using a hybrid front-tracking/level-set method (Shin {\it et al.}, J. Comp. Phys., 359, 409-435, 2018) over a wide range of surfactant properties including diffusivity, elasticity, and solubility. The effect of these properties on the speed of bubble propagation and interfacial shape is examined, paying particular attention to the bubble tail dynamics at high Reynolds number. [Preview Abstract] |
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