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 L36: Microscale Flows: Emulsions |
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Chair: Bud Homsy, University of British Columbia Room: 618 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L36.00001: Effect of adding dispersant on the structures of water-in-oil emulsions. Diego F. Muriel, Joseph Katz It is believed that dispersant application to break up marine oil slicks is effective only before stable water-in-oil emulsions develop. Both breaking waves and gentle mixing produce stable emulsions, and surfactants present in crude oil stabilize entrained sea water even in calm seas. Once formed, the interaction of these emulsions with chemical dispersant is poorly understood, partially because field observation at microscopic scales is challenging. This study examines the effect of adding dispersant (Corexit 9500) on the structure of emulsions with or without external mechanical energy. Microscopic imaging examines the size, spatial distribution, and time evolution of water droplets in the emulsion prior to and after introducing dispersant. Initially, these droplets form a multi-scale lattice with small droplets aggregating around large ones. Adding dispersant without mixing generates secondary flows as the water droplets coalesce. In time, part of the water separates, a fraction forms a cloud of submicron droplets, and the rest remains unchanged. Agitating the dispersant-emulsion mixture enhances the phase separation, removing about 66{\%} of the entrained water, and leaving an emulsion with finer droplets with different rheological properties (higher viscosity). [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L36.00002: Droplet coalescence using a microfluidic hydrodynamic trap Shweta Narayan, Davis Moravec, Brad Hauser, Andrew Dallas, Cari Dutcher Coalescence of micrometer-sized droplets to form larger drops is a fundamental process leading to separation of complex emulsions. Single droplet coalescence experiments are challenging, particularly with micrometer-sized droplets, compared to bulk studies. We have shown using microfluidics that the dynamic interfacial tension equilibration timescale is orders of magnitude shorter in micrometer-scale droplets compared to large millimeter-sized drops. Here we employ a microfluidic hydrodynamic trap to trap and coalesce single micrometer-sized droplets formed on-chip using feedback pressure control. Similar to the macro-scale four-roll mill, single droplet coalescence experiments are conducted using a microfluidic trap, with precise control over droplet size and speed. The systems studied are light and heavy mineral oils with varying concentrations of SPAN 80 in the continuous phase, with water as the dispersed phase. Film drainage times are measured as a function of Capillary number, surfactant concentration and viscosity ratio. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L36.00003: Study on the lateral migration of a ferrofluid droplet in a plane Poiseuille flow under uniform magnetic fields Md. Rifat Hassan, Cheng Wang Droplet dispersion in another immiscible fluid is important in a variety of technological processes that involve liquid-liquid extraction where phase separation is crucial to the purification of the product. In this study, we investigate the lateral migration of a ferrofluid droplet in a plane Poiseuille flow under uniform magnetic fields by means of a numerical simulation, which uses a level set method to track the droplet interface between the two phases. Focusing on low droplet Reynolds number (i.e.,${Re}_{d}\le 0.05$ ), the results indicate that the magnetic field plays a pivotal role in the motion trajectory of the droplet and the final equilibrium position in the channel. When the magnetic field acts in a direction parallel to the flow direction (i.e., $\alpha =\thinspace 0^{^{\circ}})$, the droplet settles closer to the bottom wall with increasing magnetic field strength, while at $\alpha =\thinspace {45}^{^{\circ}}$ the droplet settles closer to the center. Also, variation of initial droplet sizes results in different equilibrium positions along the channel. Furthermore, at $\alpha =\thinspace {90}^{^{\circ}}$ the droplet finds its equilibrium position at the channel center irrespective of different magnetic field strengths and droplet sizes. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L36.00004: Bubble Nucleation and Streaming in Degassed Water Kyoko Namura, Shunsuke Okai, Samir Kumar, Motofumi Suzuki Marangoni flow around a microbubble has been extensively studied to apply it for microfluidic control. However, precise control of the Marangoni flow is not straightforward because of the difficulty in controlling temperature distribution around the tiny bubble. Especially, the bubble growth under heating affects the temperature gradient on the bubble surface, which is generally induced by the diffusion of the dissolved air gases into the bubble. Here we study the bubble nucleation and subsequent flow generation in well-degassed water. The ultrapure water was sonicated under vacuum (5 kPa) to eliminate the dissolved gases. In order to realize the localized heating of the prepared degassed water, we used the photothermal effect of the gold nanoisland film. By irradiating CW laser on the film immersed in the degassed water, a water vapor bubble was generated. The bubble oscillated at sub-MHz, where its maximum size was 10 $\mu $m. The bubble involved significantly rapid flow of the order of 1 m/s under continuous photothermal heating, which is expected to be useful for microfluidic mixing and pumping. The rapid flow generation is attributed to the Marangoni force and bubble oscillation, which are induced under the steep temperature gradient. [Preview Abstract] |
Monday, November 25, 2019 2:37PM - 2:50PM |
L36.00005: Interfacial Behavior of Surfactant Covered Double Emulsion in Extensional Flow Heemin Lee, Youngwoo Kim, Susie Ryu, Joonsang Lee Surfactants significantly influence on the deformation, breakup and release of droplets. In this study, we investigated the interaction between the inner and outer interfaces of a surfactant covered double emulsion by using a three-dimensional lattice Boltzmann method. Simulations are run under 3D extensional flow. As the surfactant was concentrated on the tip, droplet was more easily deformed. We changed the initial surfactant concentration of the inner droplet and surfactant distribution of the outer droplet was delayed as the concentration increased. This result showed the effect of the inner droplet structure and Marangoni flow, which arises from the interfacial tension gradient, on external droplet surface. Next, high deformation was applied to the double emulsion and the breakup process of droplet was observed. The surfactant covered double emulsion was more stable in breakup and had elasticity to recover to its initial form. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L36.00006: Using machine learning to discover shape descriptors for predicting emulsion stability in a microfluidic channel Jian Wei Khor, Neal Jean, Chien-Yi Chang, Stefano Ermon, Sindy Tang In concentrated emulsions, drop shapes at the micro-scale carry important information about local forces and their interactions with the local environment, which can be related to its bulk properties. However, the shape descriptors used in prior work on single drops and dilute emulsions, where inter-drop interactions are minimal, are insufficient to capture the broad range of drop shapes in a concentrated system. To solve this problem, we design a convolutional autoencoder that learns to discover a low-dimensional code to describe drop shapes within a concentrated emulsion of monodisperse drops and predict whether the drop becomes unstable and undergo break-up. The model is able to faithfully reconstruct drop shapes, as well as achieve a classification accuracy of 91.7{\%} in drop break-up prediction, compared with $\sim $60{\%} using conventional scalar descriptors based on drop elongation. Finally, the method presented is expected to facilitate follow-on work to identify the relationship between drop shapes and drop interactions, and to identify potentially new modes of break-up mechanisms in concentrated system. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L36.00007: Evaporation-triggered ouzo effect in a Hele-Shaw cell Ricardo Arturo Lopez De La Cruz, Noor Schilder, Xuehua Zhang, Detlef Lohse The ouzo effect, a process of spontaneous emulsification in ternary liquid mixtures, displays unexpected phenomena when confined in a Hele-Shaw cell. Namely, branch-like patterns composed of nucleated droplets can appear under many different conditions, showing a remarkable universality [Lu Z, et al., PNAS, 114, 10332, (2017)]. In this work we further explore the droplet pattern formation due to the ouzo effect in a Hele-Shaw cell, but this time triggered by selective evaporation of the good solvent. We observed that the branch-like patterns were hindered by a solutal Marangoni instability at the liquid-air interface for certain initial solution compositions. By increasing the initial solute concentration, we were able to go from homogeneous surface droplet formation, to bulk separation and finally recover the branch-like patterns. Comparison of these phenomena with the case of an evaporating binary system allowed us to understand the initial instability as the result of the different surface tensions and volatilities between the two evaporating liquids. Furthermore, this led us to understand the regions of droplet formation and phase separation in the ternary system as regions where the instability caused accumulation of the poor solvent. [Preview Abstract] |
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