Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A36: Suspensions: Confined Flows |
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Chair: Roseanna Zia, Stanford University Room: Georgia World Congress Center B408 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A36.00001: Universal behavior of mirror-symmetric particles in confined Stokes flow Rumen Nikolaev Georgiev, Bram Bet, Sela Samin, William E Uspal, Rene van Roij, Burak Eral Deeper understanding of particle motion in microfluidic devices would unlock novel technologies for continuous, high throughput separation of distinctly-shaped particles such as cells and crystal polymorphs. Particles interact hydrodynamically with each other or with confining walls, thus altering their trajectory. These hydrodynamic interactions strongly depend on particle shape and can be tuned to guide a particle along a specific path. We focus on particles with a single mirror plane, produced via stop flow lithography in a shallow, quasi-2D microfluidic channel. A particle lags the surrounding flow due to strong confinement from the horizontal channel walls. Regardless of their exact shape, all studied particles have identical modes of motion: exponentially-decaying in-plane rotation and cross-streamline migration along a bell-shaped path. Both modes are characterized by timescales, unique to a particle’s geometry. Scaling rotation and translation of all considered shapes by their respective timescales, yields a single trajectory, universal to mirror-symmetric particles. We propose a minimalistic qualitative model capable of predicting the trajectory of various particles with one mirror plane in a shallow microfluidic flow. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A36.00002: Experimental investigation of viscoelastic effects in wavy-microchannel flow Simon J Haward, Jacob Page, Tamer A Zaki, Amy Q Shen We examine Newtonian and viscoelastic flows through slit microchannels depth d with a sinusoidal wall profile amplitude A<< d, wavelength λ and a range of dimensionless depths 0.6< α = 2πd/λ <10. Dilute solutions of high molecular weight polymer in solvents of various viscosity provide fluids with elasticity numbers 0.001<El <44. Flow velocimetry is performed in the channels over a range of flow rates. For all fluids, the wavy wall causes small perturbations (~3 %) to the Poiseuille base flow. A subtle interplay between El, α, and the imposed flow rate affects the form of the perturbation and its depth of penetration P into the channel. Our experiments support recent theoretical predictions of a ‘critical layer’ in the channel located at a dimensionless depth Σ~El0.5. Our results are consistent with the existence of a phase diagram in α-Σ parameter space showing three regimes classified as “shallow-elastic”, “deep-elastic” and “transcritical”. In the transcritical regime (defined by α > Σ, Σ < 1), the critical layer drives a surprising nonlocal amplification of the perturbation, significantly influencing P. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A36.00003: Clogging of suspensions in bidirectional flow and through obstacle arrays Brian Utter, Emily Hobbs, Chris Barber, John Buggeln, Weilun Tan The flow of granular materials, such as a suspension of macroscopic particles through a network of pores, can lead to clogging relevant to a variety of environmental and industrial flows. We perform experiments using two geometries to investigate dynamics in non-Brownian suspension clogging. In one, we characterize the interaction of grains in a bidirectional suspension flow in which two species travel in opposite directions through a channel and clogging occurs as one species impedes the flow of the other. In the other, transparent PDMS channels are used to transport quasi-2D millifluidic particle suspensions through a network of fixed pillars to characterize the spatial and temporal clogging behavior as obstacle spacing is varied. For bidirectional flow, we measure a sigmoidal clogging probability as a function of total number of grains. Data for varying channel widths collapses indicating clogging occurs at a critical density with stability decreasing with width of the channel. In the millifluidic geometry, we characterize the evolution of clogs and resulting flow in the vicinity of the obstruction via particle tracking. While the number of particles captured increases in time, non-uniform clogging leads to increased fluid velocity, particle streams, and slower accretion. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A36.00004: In-situ polyelectrolyte-clay flocculation via radial polymer injection in Taylor-Couette flows Athena Metaxas, Nikolas A Wilkinson, Ellie Raethke, Cari Dutcher Flocculation of small particulates suspended in solutions is a key process in many industries, including drinking water treatment. The particles are aggregated during mixing to form larger aggregates through use of a flocculant. The flocculation of these particulates in water treatment, however, are subject to a wide spatial variation of hydrodynamic flow states, which has consequences for floc size, growth rate, and microstructure. Floc assembly dynamics are explored here using a commercially available cationic polyacrylamide flocculant and anisotropic Na-bentonite clay under a variety of hydrodynamic mixing conditions. A Taylor-Couette cell with the unique ability to radially inject fluid into the rotating annulus was used to study how specific hydrodynamic flow fields affect floc assembly and structure during the entire flocculation process. Faster floc growth rates and decreased floc fractal dimensions were observed for higher order flow states, indicating improved mass transfer of the polymer flocculant and shear rounding of the flocs, respectively. This work sheds more light on the complexities of polymer-induced flocculation, towards improving dosing for more efficient large-scale operations. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A36.00005: On flow, fracture and getting jammed – Failure modes in dense suspensions Domenico Campanaro, Leopold Beuken, Ivo R Peters, Irmgard Bischofberger Dense suspensions are a class of complex fluids that exhibit both shear-thickening and shear-jamming behavior as a response to an applied stress. These dynamic liquid-to-solid transitions have important consequences for the displacement of a dense suspension by another fluid: upon the injection of air, intricate patterns arise in the suspension, leading to flow or fracture of the material. We displace a cornstarch suspension by a pressure controlled injection of air in a quasi-2D geometry. Depending on the concentration of cornstarch and the applied stress, we observe a variety of patterns: smooth fingering in the fluid regime and various modes of fractures, ranging from slow branched cracks to single fast fractures. Remarkably, there is a regime where, despite the application of pressure, the suspension cannot be displaced. Only upon an increase to a higher pressure, air injection occurs, leading to very thin fractures in the suspension. We hypothesize that in this regime the suspension is in the shear-jammed state. This would imply a novel way of investigating the mechanical properties of a shear-jammed material by probing their fracture behavior. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A36.00006: Effect of confinement on the dynamics of an active suspension of Quincke rotors Gerardo Pradillo, Hamid Karani, Petia M Vlahovska The electrically induced rotation of a sphere in an applied uniform DC field (Quincke effect) has been exploited to engineer particle self-propulsion (Bricard et al, Nature (2013)), by converting rotational to translational motion when the particle is sitting on a surface. In this talk, we discuss the underlying mechanism for self-propulsion in Quincke rollers, allowing us to engineer a novel system consisting of hovering Quincke rotors. We study the characteristic dynamics of these single Quincke “gliders” by quantifying the thresholds for levitation and identifying the ranges for stable stationary electro rotation, periodic rotation and onset of chaotic motion. We further introduce the emergence of unprecedented collective behavior in the multiple hovering particle system and present the corresponding phase diagram of the emergent structures. Our experimental observations offer new insights into the unexplored dynamics of the Quincke system and suggest new paths for further theoretical and experimental development in the dynamics of active systems. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A36.00007: Influence of Mechanical Degradation on Polymer Drag Reduction Zeeshan Saeed, Yasaman Farsiani, Brian R Elbing Addition of dilute (~ 10 ppm) polymer solutions can reduce the skin friction by up to 75% in both internal and external flows. The higher the molecular weight, lower is the required concentration for maximum drag reduction, but more susceptible is the polymer to chain scission in shear flows and passage constriction. Therefore, it is also important to see if skin friction reduction is sensitive to the mechanical degradation of the polymer. Thus, the current study makes a two-fold comparison; it compares the Fanning friction factor from pipe flows and then the slope increment analysis using the resulting Prandtl-Karman plots of the degraded and non-degraded polymer samples. This was performed using polyethylene oxide (PEO) solutions of 2 and 4 million grams/mol. The 4 million grams/mol sample was degraded to 2 million grams/mol by passing the sample through a contraction. The degraded molecular weight was confirmed based on the onset of drag reduction, which is inversely related to the molecular weight for PEO. This presentation will report the findings from these experiments. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A36.00008: Confinement Effect on Dip-Coating of Yield-Stress Fluids Wilbert J Smit, Christophe Kusina, Jean-François Joanny, Annie Colin Dip-coating is a widely-used industrial process in which surfaces of solid objects are covered by a thin layer of liquid. The thickness of the coating generally depends on both fluid and geometrical properties and its mastery is of major interest. Albeit the problem is well-studied for simple liquids, it remains largely elusive for yield-stress liquids—in spite of the industrial relevance. We study the effect of geometry by withdrawing rods from cylindrically-shaped baths filled with a Carbopol gel (yield stress 4—130 Pa). The coating thickness is observed to strongly depend on the geometrical confinement. The thickness decreases with decreasing separation between the rod and the sides of the bath. This confinement effect is stronger for high yield stress. A smaller rod is observed to result in thinner coatings. We will discuss and interpret these results. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A36.00009: Influence of channel size on velocity profiles of shear banding fluids in pressure driven flow Paul Salipante, Alexander Conte, Vishnu Dharmaraj, Steven David Hudson We investigate the effect of channel dimensions on the velocity profiles of shear banding wormlike micelle solutions in pressure driven microcapillary flow. Velocity profiles are measured using holographic 3D particle tracking velocimetry on the fluids seeded with microparticles. The velocity profiles at different volumetric flow rates are compared to expected velocity profiles determined from rotational rheometry. Capillary rheology measurements are performed simultaneously via measurements of flow rate and pressure drop across the capillary. Results from the capillary rheology measurements yield viscosity values consistent with rotational rheometry. Although, we observe deviation between the measured velocity profile and that expected from the global rheology measurements. We attribute the deviation to molecular diffusion, which produces a finite interfacial width between the two shear bands and leads to a rapid decrease in shear rate near the wall. We compare our velocity profile results to theoretical models that include a diffusive terms in the stress balance, which becomes relevant at small channel sizes. |
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