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 L35: Microscale Flows: Nonnewtonian Fluids |
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Chair: Simon Haward, Okinawa Institute of Science and Technology Room: 617 |
Monday, November 25, 2019 1:45PM - 1:58PM |
L35.00001: Cross-stream migration of non-spherical particles in a second-order fluid Shiyan Wang, Cheng-Wei Tai, Vivek Narsimhan Particle migration in viscoelastic suspensions is vital in many applications in the biomedical community and the chemical/oil industries. Despite previous studies on the motion of spherical particles in simple viscoelastic linear flows, the combined effect of more complex flow profiles and particle shapes is underexplored. Here, we study the dynamics of arbitrary-shaped particles in a second-order fluid, subject to a general quadratic flow field. For the two model constants $\psi $1 and $\psi $2 (first and second normal stress coefficients) we assume the relationship $\psi $1$=$-2$\psi $2. The assumption allows us apply a multipole expansion to derive analytical expressions for the polymeric force and torque of the particle. We apply the analytical solutions to track the translational and rotational trajectories of spheres, spheroids, and general ellipsoids in shear and pressure driven flows. In shear flows, we observe that prolate-like particles undergo a transition from tumbling to log-rolling motion (i.e., alignment along the vorticity direction) as the shear rate increases. At very large shear rates, the particles can reorient along the flow direction, but this state is metastable. In pressure driven flows, we find that particles migrate to the center of the flow, with the tumbling period increasing in time until the particle eventually aligns along the flow direction. [Preview Abstract] |
Monday, November 25, 2019 1:58PM - 2:11PM |
L35.00002: Viscoelasticity based droplet migration and sorting. Shamik Hazra, Sushanta Mitra, Ashis Kumar Sen We experimentally elucidate the cross-stream migration behavior of viscoelastic Polydimethylsiloxane (PDMS) droplets in the non-inertial Poiseuille flow of constant viscosity aqueous viscoelastic solution of Polyvinylpyrrolidone (PVP) in straight rectangular microchannels. We investigate the complex interaction among deformability induced non-inertial lift force, viscoelastic lift force due to matrix viscoelasticity, and viscoelastic lift force due to droplet viscoelasticity and propose a new droplet migration regime. We vary the drop-to-medium viscosity ratio (k) by varying the PVP concentration and demonstrate center-ward droplet migration for k \textless 18.52 and \textgreater 3.72 respectively, while droplets stay near the wall for 3.72$\le $ k $\le $18.52, contrary to Chan and Leal's prediction with second-order fluids. We observe that for k $=$ 12, Newtonian castor oil (viscosity 650 mPa-s, relaxation time $=$ 0) droplets migrate to center while PDMS (average viscosity 666 mPa-s, relaxation time $=$ 0.001) droplets stay near the wall. Finally, we demonstrate viscoelasticity based sorting of castor and PDMS droplets. [Preview Abstract] |
Monday, November 25, 2019 2:11PM - 2:24PM |
L35.00003: Oscillations of a cantilevered micro beam driven by a viscoelastic flow instability Anita Dey, Yahya Modarres-Sadeghi, Anke Lindner, Jonathan Rothstein We will present the results of our study of the flow of a viscoelastic polymer solution past a cantilevered beam in a confined geometry. The flow of viscoelastic fluids, unlike Newtonian fluids, can become unstable even at infinitesimal Reynolds numbers due to purely elastic flow instabilities that occur at large Weissenberg numbers. With increasing Weissenberg number, we will show that elastic instabilities occur in the vicinity of a flexible beam and begin to interact with the beam. We will report these interactions for cantilevered beams with varying elastic modulus, beam length and rigidity. Over a range of Weissenberg numbers, we will show that the flow field transitions from a stable detached vortex upstream of the beam to a time-dependent unstable vortex shedding. The shedding of the unstable vortex upstream of the beam will be shown to couple with the flow-induced beam deformation triggering oscillations of the beam. The critical onset of the flow transitions, mechanism of vortex shedding, and dynamics of the cantilevered beam oscillations will be presented for beams with varying flexibility. The oscillations of the flexible beam will be shown to have distinct regimes: a clear single vortex shedding regime and another regime characterized by a 3D chaotic flow instability. [Preview Abstract] |
Monday, November 25, 2019 2:24PM - 2:37PM |
L35.00004: Two ball interaction while settling in an Oldroyd-B fluid: one atop the other initially Tsorng-Whay Pan, Shang-Huan Chiu, Roland Glowinski In this talk we present a numerical study of two ball interaction while settling in a vertical channel with a square cross-section filled with an Oldroyd-B fluid. Two balls are released one atop the other initially and the effects of particle inertia and fluid inertia are not ignored. We have obtained that either the trailing ball catches up the leading one to form a chain or two balls separate with a stable final distance at the end. For the cases of the ball density slightly heavier than that of the fluid, they can form a vertical chain or tilted chain. But when increasing the ball density, the two balls can form a chain for smaller initial gaps; but they move away from each other and the distance between two balls reaches a constant for a larger initial gap at higher elasticity number. Thus the two ball chain formation is up to, at least, the ball settling speed (i.e., the ball density), initial gap between two balls and elasticity number. Also the fluid polymer extension limit has its effect on the formation of two ball chain when they settle in a FENE type of viscoelastic fluid. [Preview Abstract] |
(Author Not Attending)
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L35.00005: Filament thinning dynamics of Boger fluids in extensional flow under microfluidic environment Tanoy Kahali, Suman Chakraborty Polymer addition to Newtonian-fluid drastically alters the neck thinning dynamics especially in reduced length scale system due to enhanced elasticity. This motivates to study the neck thinning of two-phase polymeric--Newtonian system in micro-scale. Previous studies are mostly focused on gravity assisted filament thinning in macro-scale. Here, we intend to study the effect of elasticity on filament thickness for a series of Boger fluids(dispersed phase)stretched in a medium of silicon oil under micro-environment.Qualitatively, we observed that the filament thinning rate is much slower compared to Newtonian fluid. At an earlier stage, both fluids undergo a shear thinning process where the filament thickness decays exponentially with time. In later stage, for Newtonian fluid a capillary driven regime dictates further thinning and rupture of the filament. In contrast, a second exponential thinning regime (causing the delay) is observed for polymer filaments along with capillary-driven thinning before pinch off. We envisage that this analysis may elucidate the role of different types of polymer addition and its concentration on the universal trend of filament thinning process in micro-scale. [Preview Abstract] |
Monday, November 25, 2019 2:50PM - 3:03PM |
L35.00006: Viscoelastic fluid-structure interactions in microfluidics Simon Haward, Cameron Hopkins, Amy Shen Flow of a viscoelastic wormlike micellar solution around a slender, but rigid, microfluidic post at negligible Reynolds number (Re\textless \textless 1) undergoes a supercritical bifurcation to a steady asymmetric state when a critical Weissenberg number (Wi) is exceeded. A second transition above a higher critical Wi results in time-dependence of the asymmetric flow with a characteristic frequency 1/$\lambda_{\mathrm{M}}$, where $\lambda_{\mathrm{M}}$ is the Maxwellian relaxation time. We examine the effect of this time dependence on the behavior of flexible cantilevered micro-posts, showing post oscillations at the same characteristic frequency 1/$\lambda_{\mathrm{M}}$, thus demonstrating a ``purely-elastic'' fluid-structure interaction. A second flexible post positioned downstream, shows a remarkable degree of synchronization with the first. The time lag between their correlated motions is much shorter than any expected flow time between the two posts. Our experiments show that the posts are effectively linked by an elastic strand of highly stressed fluid originating in the wake of the upstream post. Our results indicate that the time lag between their motion is dictated by the speed of the elastic wave traveling along this strand. [Preview Abstract] |
Monday, November 25, 2019 3:03PM - 3:16PM |
L35.00007: Motion of nanoparticle-covered droplet in a square microchannel Zhengyuan Luo, Bofeng Bai The flow of complex droplets with contaminated surfaces (e.g., nanoparticle-covered interface) in microchannels with non-circular cross-sections is ubiquitous in nature and various engineering applications, for example, deformable droplets through porous media in underground oil reservoir and droplet transport and manipulation in microfluidic devices. It is also an important fundamental question in the discipline of fluid mechanics. Extensive studies have been dedicated to the study of the motion, deformation and breakup of an individual droplet in cylindrical capillaries or non-circular channels, most of which have been focused on clean droplets. However, little is known about the effects of contaminated surfaces, e.g., nanoparticle-covered interface. In this study, we will show our new numerical results on flow dynamics of complex droplets with nanoparticle-covered interface in a square microchannel. We find the adsorption of nanoparticles tends to assemble at the drop rear and immobilize the drop surface, and thus enlarges the droplet-induced extra pressure loss. [Preview Abstract] |
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