Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M25: Microscale Flows: Emulsions and Mixing |
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Chair: Shelley Anna, Carnegie Mellon University Room: E145 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M25.00001: The Role of Colloidal Interactions on the Formation of Particle Stabilized Capsules Shelley Anna, Charles Sharkey, Anthony Kotula Nanoparticles can adsorb to fluid-fluid interfaces to make stable foams and emulsions. Surfactants adsorbed to the nanoparticle surface modulate both particle wettability and interparticle interactions, altering the nanoparticle adsorption. We have shown that bubbles generated in a nanoparticle-surfactant mixture collect particles as they travel through a long microchannel. The particle stabilized region of the bubble grows in a manner consistent with convection and diffusion of particles in the fluid surrounding the bubble. If the bubble residence time is long enough compared with the adsorption timescales, a stable, non-spherical, gas-filled capsule emerges from the microchannel and retains its shape for tens of hours. We find that the nanoparticle-surfactant mixture composition can be used to tune the degree of capsule stabilization. Greater stabilization occurs with larger surfactant concentrations for a fixed nanoparticle volume fraction. These observations can be rationalized in terms of the particle wettability and electrostatic interactions as well as interfacial elasticity and bulk nanoparticle transport and adsorption. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M25.00002: Compact and controlled microfluidic mixing and biological particle capture Matthew Ballard, Drew Owen, Zachary Grant Mills, Peter J. Hesketh, Alexander Alexeev We use three-dimensional simulations and experiments to develop a multifunctional microfluidic device that performs rapid and controllable microfluidic mixing and specific particle capture. Our device uses a compact microfluidic channel decorated with magnetic features. A rotating magnetic field precisely controls individual magnetic microbeads orbiting around the features, enabling effective continuous-flow mixing of fluid streams over a compact mixing region. We use computer simulations to elucidate the underlying physical mechanisms that lead to effective mixing and compare them with experimental mixing results. We study the effect of various system parameters on microfluidic mixing to design an efficient micromixer. We also experimentally and numerically demonstrate that orbiting microbeads can effectively capture particles transported by the fluid, which has major implications in pre-concentration and detection of biological particles including various cells and bacteria, with applications in areas such as point-of-care diagnostics, biohazard detection, and food safety. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M25.00003: Investigating droplet internal flow in concentrated emulsion when flowing in microchannel using micro-PIV Chia Min Leong, Ya Gai, Sindy K. Y. Tang Droplet microfluidics has enabled a wide variety of high throughput applications through the use of monodisperse droplets. Previous fluid studies of droplet microfluidics have focused on single drops or emulsions at low volume fractions. The study of concentrated emulsions at high volume fractions is important for increasing the throughput, but the fluid dynamics of such emulsions in confined channels is not well understood. Here we describe two-dimensional, mid-height measurements of the flow inside individual drops within a concentrated emulsion using micro-PIV. The emulsion has 85{\%} volume fraction and flows as a monolayer in a straight microfluidic channel. The effects of confinement and viscosity ratio on the internal flow patterns inside the drops were studied. The results show rotational structures inside the drops always exist, and are independent of viscosity ratio for the conditions tested. The structures depend on droplet mobility which in turn, depends on the confinement of the emulsion and the location of the drops in the channel. To our best knowledge, no work has probed the flow field inside droplets of concentrated emulsions at high volume fractions in confined channels. Current work is in progress to measure the three-dimensional flow field in such system. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M25.00004: Periodic dislocation dynamics in two-dimensional concentrated emulsion flowing in a tapered microchannel Ya Gai, Chia Min Leong, Wei Cai, Sindy K. Y. Tang Here we report a surprising order in concentrated emulsion when flowing as a monolayer in a tapered microfluidic channel. The flow of droplets in micro-channels can be non-trivial, and may lead to unexpected phenomena such as long-period oscillations and chaos. Previously, there have been studies on concentrated emulsions in straight channels and channels with bends. The dynamics of how drops flow and rearrange in a tapered geometry has not yet been characterized. At sufficiently slow flow rates, the drops arrange into a hexagonal lattice. At a given x-position, the time-averaged droplet velocities are uniform. The instantaneous drop velocities, however, reveal a different, wave-like pattern. Within the rearrangement zone where the number of rows of drops decreases from N to N-1, there is always a drop moved faster than the others. Close examination reveals the anomalous velocity profile arises from a series of dislocations that are both spatial and temporal periodic. To our knowledge, such reproducible dislocation motion has not been reported before. Our results are useful in novel flow control and mixing strategies in droplet microfluidics as well as modeling crystal plasticity in low-dimensional nanomaterials. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M25.00005: Phonons in active microfluidic crystals Alan Cheng Hou Tsang, Eva Kanso One-dimensional crystals of driven particles confined in quasi two-dimensional microfluidic channels have been shown to exhibit propagating sound waves in the form of `phonons', including both transverse and longitudinal normal modes. Here, we focus on one-dimensional crystals of motile particles in uniform external flows. We study the propagation of phonons in the context of an idealized model that accounts for hydrodynamic interactions among the motile particles. We obtain a closed-form analytical expression for the dispersion relation of the phonons. In the moving frame of reference of the crystals, the traveling directions of the phonons depend on the intensity of the external flow, and are exactly opposite for the transverse and longitudinal modes. We further investigate the stability of the phonons and show that the longitudinal mode is linearly stable, whereas the transverse mode is subject to an instability arising from the activity and orientation dynamics of the motile particles. These findings are important for understanding the propagation of disturbances and instabilities in confined motile particles, and could generate practical insights into the transport of motile cells in microfluidic devices. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M25.00006: Microfluidic step-emulsification in a cylindrical geometry Indrajit Chakraborty, Alexander M Leshansky The model microfluidic device for high-throughput droplet generation in a confined cylindrical geometry is investigated numerically. The device comprises of core-annular pressure-driven flow of two immiscible viscous liquids through a cylindrical capillary connected co-axially to a tube of a larger diameter through a sudden expansion, mimicking the microfluidic step-emulsifier (1). To study this problem, the numerical simulations of axisymmetric Navier-Stokes equations have been carried out using an interface capturing procedure based on coupled level set and volume-of-fluid (CLSVOF) methods. The accuracy of the numerical method was favorably tested vs. the predictions of the linear stability analysis of core-annular two-phase flow in a cylindrical capillary. Three distinct flow regimes can be identified: the dripping (D) instability near the entrance to the capillary, the step- (S) and the balloon- (B) emulsification at the step-like expansion. Based on the simulation results we present the phase diagram quantifying transitions between various regimes in plane of the capillary number and the flow-rate ratio. (1) Z. Li et al., Lab on a Chip 15, 1023 (2015). [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M25.00007: A Novel Miniaturized Mixer Based on a Wankel Geometry Pankaj Kumar, Stephen Wan Mixing in microfluidic systems is a challenge since the flow regime encountered in these systems is typically very low Reynolds number laminar flow, in which viscous forces dominate inertial forces, which precludes efficient turbulence-based mixing. Mixing based purely on diffusion is also not a practical alternative due to the long times required to achieve a sufficient level of mixing. The present study presents a pump based on Wankel geometry as a mixer for efficient mixing in a microfluidic system. Then, a novel modification to the internal geometry of the Wankel-pump-mixer is analyzed and is shown to enable robust mixing without the introduction of an additional system component and hence without the expense of undesirable dead volume. The Lagrangian Coherent Structures (LCS) calculated from the Finite-Time Lyapunov Exponent (FTLE) field with a mixing measure is used to quantify the mixing. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M25.00008: Design and optimization of anode flow field of a large proton exchange membrane fuel cell for high hydrogen utilization Serhat Yesilyurt, Omid Rizwandi We developed a CFD model of the anode flow field of a large proton exchange membrane fuel cell that operates under the ultra-low stoichiometric (ULS) flow conditions which intend to improve the disadvantages of the dead-ended operation such as severe voltage transient and carbon corrosion. Very small exit velocity must be high enough to remove accumulated nitrogen, and must be low enough to retain hydrogen in the active area. Stokes equations are used to model the flow distribution in the flow field, Maxwell-Stefan equations are used to model the transport of the species, and a voltage model is developed to model the reactions kinetics. Uniformity of the distribution of hydrogen concentration is quantified as the normalized area of the region in which the hydrogen mole fraction remains above a certain level, such as 0.9. Geometry of the anode flow field is modified to obtain optimal configuration; the number of baffles at the inlet, width of the gaps between baffles, width of the side gaps, and length of the central baffle are used as design variables. In the final design, the hydrogen-depleted region is less than 0.2{\%} and the hydrogen utilization is above 99{\%}. [Preview Abstract] |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M25.00009: Parametric study on phase separation of binary mixtures in a lid driven cavity: A DPD study Harinadha Gidituri, Vijay Anand, Mahesh Panchagnula, Srikanth Vedantam We investigate the phase separation behavior of binary mixtures in two dimensional periodic and lid driven cavity domains using dissipative particle dynamics (DPD). The effect of DPD parameters like repulsion coefficient, dissipative coefficient, cut-off radius, and weight function exponent on domain size growth has been studied. The phase separation is delayed for low values of repulsion coefficient. Under these conditions, a few clusters of the dispersed phase are distributed in a continuous phase. This is because of weak inter-particle repulsion. As we increase the repulsion coefficient value, this behavior disappears. The domain growth rate is also observed to increase with an increase in the value of the dissipation coefficient as well as cut-off radius. Finally, the dynamics of phase separation in the lid driven cavity problem are significantly different when compared to that in the periodic domain, due to the formation of a stable vortex in the cavity. The vortex results in a dynamic equilibrium between clustering and separation. The distribution of cluster sizes is studied as a function of the driven cavity parameters. [Preview Abstract] |
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