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 E35: Drops: Complex Fluids |
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Chair: Jan Guzowski, Princeton University Room: Ballroom B |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E35.00001: Gel-like double-emulsion droplets Jan Guzowski, Piotr Korczyk, Piotr Garstecki, Howard Stone We experimentally study the problem of packing of micro-droplets inside a droplet of another immiscible liquid phase. We use microfluidics to encapsulate multiple monodisperse aqueous segments inside a drop of oil. For small numbers N (N\textless 10) of the aqueous droplets and at their volume fraction in oil exceeding the close-packing threshold we observe multiple metastable structures with well-defined point-group symmetries. We attribute the observed metastability to the deformability of the droplets which leads to effective many-body interactions and energy barriers for rearrangement. By changing the composition of the oil phase we find that when the surface tensions of the droplets and of the encapsulating phase are comparable, the energy barriers are high enough to trap elongated structures or even linear chains, independently of N. However, when the surface tension of the encapsulating phase is much larger than that of the droplets, non-spherical morphologies are stable only at sufficiently high N. In such a case multiple internal interfaces can hold stresses and prevent relaxation of the global deformations which leads to a plastic, gel-like behavior. Our findings can serve as guidelines for synthesis of functional particles as well as for designing biomimetic materials, e.g. for tissue engineering. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E35.00002: Yield-stress fluid drop impact on heated surfaces Brendan Blackwell, Alex Wu, Randy Ewoldt Yield-stress fluids, including gels and pastes, are effectively fluid at high stress and solid at low stress. In liquid-solid impacts, these fluids can stick and accumulate where they impact, motivating several applications of these rheologically-complex materials. Here we use high-speed imaging to experimentally study liquid-solid impact of yield-stress fluids on heated surfaces. At low temperatures yield-stress fluids tend to stick to surfaces and leave a coating layer. At sufficiently high temperatures the Leidenfrost effect can be observed, wherein a layer of vapor is created between the material and the surface due to rapid boiling, which can prevent a droplet of yield-stress fluid from sticking to the surface. In this study rheological material properties, drop size, drop velocity, and surface temperature are varied to characterize behavioral regimes. Material sticking to and releasing from the surface is observed as a function of the input parameters. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E35.00003: A Computational Study of the Rheology and Structure of Surfactant Covered Droplets Joao Maia, Arman Boromand, Safa Jamali The use of different types of surface-active agents is ubiquitous practice in different industrial applications ranging from cosmetic and food industries to polymeric nano-composite and blends. This allows stable multiphasic systems like foams and emulsions to be produced. Stability and shelf-life of those products are directly determined by the efficiency of the surfactant molecules. Although the effect of molecular configuration of the surface-active molecules on the planar interfaces has been studied both experimentally and computationally, it remains challenging to track the efficiency and effectiveness of different surfactant molecules on curved interfaces. In this study we address this gap by using Dissipative Particle Dynamics, to study the effectiveness and efficiency of different surfactant molecules (linear vs. branched) on a curved interface in equilibrium and far from equilibrium. In particular, we are interested to relate interfacial properties of the surface covered droplets and its dynamics to the molecular configuration of the surface active molecules under equilibrium and far from equilibrium condition. [Preview Abstract] |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E35.00004: Simulation of Droplet Generation in a Non-Newtonian Dense Granular Suspension Gustaf MÃ¥rtensson, Martin Svensson, Andreas Mark, Fredrik Edelvik As with the jet printing of dyes and other low-viscosity fluids, the jetting of dense fluid suspensions is dependent on the repeatable break-off of the fluid filament into well-formed droplets. It is well known that the break-off of dense suspensions is dependent on the volume fraction of the solid phase, particle size and morphology, fluid phase viscosity et cetera, see for example van Deen et al. (2013). The purpose of this study is to propose a novel simulation framework and to show that it captures the main effects such as droplet shape, volume and speed in a cylindrical duct test configuration. The granular suspension is modelled as a mixed single phase suspension, where the local thermodynamic properties are determined by the mixture level. The simulations are performed with IBOFlow, a multiphase flow solver, coupled with LaStFEM, a large strain FEM solver. To study how the droplet generation is affected by the acceleration of the fluid, simulations are performed for a series of actuation profiles. The simulation results were compared to experimental data obtained from an industrial jetting head. The simulations exhibit qualitative agreement with the experimental data. A sensitivity to the inlet boundary condition with respect to the resulting droplet speed was observed. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E35.00005: Multiple phenomena triggered by surfactant solutions on liquid pools XIANG WANG When a drop of aqueous surfactant solution is placed on a deep subphase of water, multiple phenomena occur. The contact line of the drop spreads until the drop merges with the subphase. A capillary wave train is initiated by the disturbance caused by the drop touching the subphase surface. Marangoni stresses cause the formation and propagation of a localized distortion of the subphase surface (subsequently called the Marangoni ridge). And particles pre-deposited on the subphase surface are propelled by the flow induced by Marangoni stresses. We examine all these phenomena simultaneously at early times. The drop contact line has the slowest propagation speed. The Marangoni ridge propagates slower than the slowest crest of the capillary wave train and the capillary wave speed is affected by the presence of the surfactant. Particle motion is not induced by the capillary waves but is initiated by the passing Marangoni ridge. The particles are rapidly accelerated by the force from surface tension gradient acting on the contact line of the particles and viscous forces acting on the submerged surface of the particles. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E35.00006: Coalescence avalanches in 2D emulsions: a stochastic approach Danny Raj Masila, Raghunathan Rengaswamy One coalescence event in a 2D concentrated emulsion can trigger an avalanche resulting in the rapid destabilization of the drop-assembly. The sensitive dependence of this phenomenon on various factors that include surfactant concentration and viscosities of the fluid phases makes the avalanching problem appear probabilistic. We propose a stochastic framework- that utilizes a probability function to explain local coalescence events- to study the dynamics of the coalescence avalanches. A function that accounts for the local coalescence mechanism is used to fit the experimentally (from literature) measured probability data. A continuation parameter is introduced along with this function to account for the effect of system properties on the avalanche dynamics. Our analysis reveals that this behavior is a result of the inherent autocatalytic nature of the process. We discover that the avalanche dynamics shows critical behavior where two outcomes are favored: no avalanche and large avalanches that lead to destabilization. We study the effect of system size and fluid properties on the avalanche dynamics. A sharp transition from non-autocatalytic (stable emulsions) to autocatalytic (unstable) behavior is observed as parameters are varied. [Preview Abstract] |
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