Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A15: Microscale Flows: Emulsions and RelatedMicro
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Chair: Grae Worster, University of Cambridge Room: 601 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A15.00001: Microfluidic generation of particle-stabilized water-in-water emulsions Niki Abbasi, Maryam Navi, Scott S. H. Tsai We present a microfluidic platform that generates particle-stabilized water-in-water emulsions, using an aqueous two-phase system (ATPS) of polyethylene glycol (PEG) and Dextran (DEX). DEX droplets are generated passively at a flow focusing junction, in a continuous phase of PEG and carboxylated particles, using weak hydrostatic pressure to drive the flow. As DEX droplets travel inside the microfluidic device, carboxylated particles partition to the interface of the droplets. The number of particles partitioning to the interface of droplets increases as the droplets migrate downstream in the microchannel. As a result, the DEX droplets become stabilized against coalescence. We study the coverage and stability of the DEX droplets further downstream inside a reservoir, by changing the carboxylated particle concentration and the particle size. We anticipate that particle-stabilized water-in-water emulsions may have important biotechnological applications, due to their intrinsic biocompatibility compared to traditional particle-stabilized water-in-oil emulsions, for example for cell encapsulation. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A15.00002: Microfluidic emulsification at cross-junction: experimental and numerical study using Blue Evangelia Roumpea, Nina M. Kovalchuk, Lyes Kahouadji, Zhihua Xie, Maxime Chinaud, Mark J.H Simmons, Omar K. Matar, Panagiota Angeli Liquid-liquid drop formation in a cross-junction device is investigated both experimentally and numerically. Experiments are performed using 5 cSt silicone oil as the continuous phase and 52\% glycerol/ 48\% water mixture containing surfactants as the dispersed phase. Both a high-speed camera and a two-colour micro-PIV technique were used to obtain the different flow regimes i.e. squeezing, dripping, jetting and threading and to study the velocity fields of the two phases simultaneously. The dependence of the drop size on flow rate follows a power law with different exponents for small and large drops. Numerical simulations using the code {\it Blue}, a massive parallel solver for simulations of fully three-dimensional multiphase flows, were also performed taking into account the properties of the liquids used in the experiments and the precise geometry of the microfluidic chips. The simulation results agreed very well with the surfactant-free solution. The numerical simulations taking into account the surfactant are ongoing. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A15.00003: Magnetic water-in-water droplet microfluidics Maryam Navi, Niki Abbasi, Scott S. H. Tsai Aqueous two-phase systems (ATPS) have shown to be ideal candidates for replacing the conventional water-oil systems used in droplet microfluidics. We use an ATPS of Polyethylene Glycol (PEG) and Dextran (DEX) for microfluidic generation of magnetic water-in-water droplets. As ferrofluid partitions to DEX phase, there is no significant diffusion of ferrofluid at the interface of the droplets, rendering generation of magnetic DEX droplets in a non-magnetic continuous phase of PEG possible. In this system, both phases are water-based and highly biocompatible. We microfluidically generate magnetic DEX droplets at a flow-focusing junction in a jetting regime. We sort the droplets based on their size by placing a permanent magnet downstream of the droplet generation region, and show that the deflection of droplets is in good agreement with a mathematical model. We also show that the magnetic DEX droplets can be stabilized by lysozyme and be used for separation of single cell containing water-in-water droplets. This system of magnetic water-in-water droplet manipulation may find biomedical applications such as single-cell studies and drug delivery. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A15.00004: Time-varying droplet configuration determines break-up probability of drops within a concentrated emulsion Jian Wei Khor, Minkyu Kim, Simon Schutz, Tobias Schneider, Sindy Tang In this study, we investigate the origin of the probability of break-up of drops within a concentrated emulsion flowing as a 2D monolayer through a tapered microchannel into a constriction. Although the concentrated emulsion is complex involving many-body interactions, all break-up events occur between two drops pinching each other as they enter the constriction under the conditions tested. Whether break-up occurs or not depends strongly on the relative position between the two drops at the entrance of the constriction. There exists a critical offset between the initial positions of the two drops below which break-up always occurs, and another critical offset above which no break-up occurs. In between these two critical offsets, there is a narrow bistable region where both break-up and non-break-up events are observed. For a flowing concentrated emulsion, the relative position between two droplets entering the constriction varies stochastically. The frequency of occurrence of drop pairs having specific offsets, together with the critical offset values for break-up, determines the break-up probability in the flowing emulsion. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A15.00005: Amphiphilic nanoparticles suppress droplet break-up and increase serial droplet interrogation rate. Minkyu Kim, Ya Gai, Ming Pan, Sindy K. Y. Tang We describe the break-up behavior of a concentrated emulsion comprising drops stabilized by amphiphilic silica nanoparticles (`NPs') flowing in a tapered microchannel. Such channel geometry is commonly used in droplet serial interrogation and assay. We focus on concentrated emulsions as they often form after the droplet incubation. Unlike solid wells in their multi-well plate counterpart, drops are prone to interfacial instability. Droplet break-up ruins assay accuracy. The rate at which break-up occurs sets the limit for assay throughput. Previously, we have studied the break-up of surfactant-stabilized drops in a concentrated emulsion. The key motivation for replacing surfactants with NPs is that NPs can mitigate inter-drop transport of small molecules. Our results show replacing surfactant with NPs as droplet stabilizers has an additional advantage of reducing droplet break-up, thereby increasing the droplet interrogation rate. Such result can be attributed to the increased interfacial viscoelasticity. We examine the effect of channel confinement, viscosity ratio, and size of NPs on the break-up behavior of drops. We find the break-up is dependent on confinement and size of NPs, while insensitive to viscosity ratio within the tested range. Our results have immediate practical use in increasing the throughput limit of droplet-based applications such as serial assay and interrogation. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A15.00006: Mixing in millimeter-scale drops by the action of surface viscosity Frank Riley, Shreyash Gulati, Amir Hirsa, Juan Lopez Mixing within drops can be challenging as it involves low Reynolds numbers. Researchers have used techniques including electrowetting, acoustic, electric, magnetic, or mechanical excitation to produce mixing in drops for applications such as sample processing, pathogen detection and controlled reactions. Here we present a new strategy, namely the action of surface shear viscosity to produce mixing in drops. This was accomplished in a drop that is constrained between two rings: one of which rotates and the other can be counter-rotated or held stationary. Rotation of the ring causes interfacial shear which in turn drives bulk flow through the action of surface shear viscosity which can lead to a strong bulk flow. FEM-based models have been developed to predict the flow physics in the ring-sheared drop. Boussinesq-Scriven surface model has been used at the air-liquid interface. Mixing was studied for several configurations of the ring-sheared drop where the rings were rotated in steady, oscillatory and counter-rotating fashions. Prediction from the new model were tested against an established model that was experimentally validated. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A15.00007: On an ordered flow of a two-dimensional concentrated emulsion confined in a tapered microchannel Ya Gai, Chia Min Leong, Wei Cai, Sindy K. Y. Tang We report an unexpected order in a monolayer flow of concentrated emulsion in a tapered microchannel. The flow of droplets in confined geometry can be non-trivial, giving rise chaos and long-period oscillations. Previously, there have been studies on concentrated emulsions in straight and bended channels. The dynamics of how a concentrated emulsion flow, especially the interactions among droplets, has not yet been characterized in a tapered geometry. Our results show at sufficiently low flow rates, while the time-averaged droplet velocities are uniform, the instantaneous drop velocities exhibit a wave-like periodicity. A close examination reveals this anomalous velocity profile arises from a sequence of rearrangement events that are both spatially and temporally periodic. We show these ordered rearrangement events can be modeled by the slipping motion of dislocations in crystal plasticity. In addition, we characterize the timescale associated with these rearrangements and identify parameters that affect this timescale. To our best knowledge, such ordered flow in a confined concentrated emulsion has not been studied before. Our results are useful in droplet-based flow control and mixing strategies as well as modeling crystal plasticity in low-dimensional nanomaterials. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A15.00008: Computation of shear-induced collective-diffusivity in emulsions Abhilash Reddy Malipeddi, Kausik Sarkar The shear-induced collective-diffusivity of drops in an emulsion is calculated through simulation. A front-tracking finite difference method is used to integrate the Navier-Stokes equations. When a cloud of drops is subjected to shear flow, after a certain time, the width of the cloud increases with the $\frac{1}{3}$ power of time. This scaling of drop-cloud-width with time is characteristic of (sub-)diffusion that arises from irreversible two-drop interactions. The collective diffusivity is calculated from this relationship. A feature of the procedure adopted here is the modest computational requirement, wherein, a few drops ($\sim$70) in shear for short time ($\sim$70 strain) is found to be sufficient to get a good estimate. As far as we know, collective-diffusivity has not been calculated for drops through simulation till now. The computed values match with experimental measurements reported in the literature. The diffusivity in emulsions is calculated for a range of Capillary ($Ca$) and Reynolds ($Re$) numbers. It is found to be a unimodal function of $Ca$, similar to self-diffusivity. A sub-linear increase of the diffusivity with $Re$ is seen for $Re<5$. This work has been limited to a viscosity matched case. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A15.00009: Absorption of charged particulate surfactants in microfluidics. tiantian kong, zhou liu, Xiaoxue Yao, Yaming Liu We use microfluidics to uncouple the generation of Pickering emulsion droplets and stability analysis against coalescence. By designing the microchannels, we control the packing time for charged particles arriving at the droplet interfaces, and subsequently test the droplet stability in a coalescence chamber. The critical particle coverage on interfaces that prevents coalescence are estimated by an adsorption model. We further investigate the dependence of the critical particle coverage on its properties such as particle sizes, surface charge densities, and bulk concentrations. Our studies are potentially beneficial to the applications involving particle-stabilized droplets including cosmetics, food products, and oil recovery. [Preview Abstract] |
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