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 M26: Suspensions: General |
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Chair: Enrique Rame, USRA Room: E146 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M26.00001: Electric field driven mesoscale phase transition in polarized colloids. Boris Khusid, Ezinwa Elele, Qian Lei A mesoscale phase transition in a polarized suspension was reported by Kumar, Khusid, Acrivos, PRL95, 2005 and Agarwal, Yethiraj, PRL102, 2009. Following the application of a strong AC field, particles aggregated head-to-tail into chains that bridged the interelectrode gap and then formed a cellular pattern, in which large particle-free domains were enclosed by particle-rich thin walls. Cellular structures were not observed in numerous simulations of field induced phase transitions in a polarized suspension. A requirement for matching the particle and fluid densities to avoid particle settling limits terrestrial experiments to negatively polarized particles. We present data on the phase diagram and kinetics of the phase transition in a neutrally buoyant, negatively polarized suspension subjected to a combination of AC and DC. Surprisingly, a weak DC component drastically speeds up the formation of a cellular pattern but does not affect its key characteristic. However, the application of a strong DC field destroys the cellular pattern, but it restores as the DC field strength is reduced. We also discuss the design of experiments to study phase transitions in a suspension of positively polarized, non-buoyancy-matched particles in the International Space Station. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M26.00002: A hydrodynamic linear instability in a system of confined colloidal rollers Blaise Delmotte, Aleksandar Donev, Michelle Driscoll, Paul Chaikin In a typical flow instability, the fastest growing wavelength is selected by two or more competing stresses. In this talk I will discuss a very different kind of instability, controlled by a single geometric parameter. We study theoretically a new instability which has been observed experimentally and numerically: the fingering of a front of suspended microrollers near a floor. Our continuum model shows that this instability is linear and that the size scale selection arises only from hydrodynamic interactions between the particles and the wall, independently of the driving forces and viscosity. We believe that this instability mechanism is quite generic and selects the instability length scale in a number of suspension/colloid systems near a wall. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M26.00003: Stability and transition to chaos of regular capsule trains Spencer Bryngelson, Jonathan Freund Elastic capsules flowing in sufficiently narrow confines, such as red blood cells in capillaries, are well-known to line up in a single-file train. The stability of such a train in less confined environments, where this organization is not observed, is investigated in a model system that includes full coupling between the viscous flow and suspended elastic capsules. A rich set of linearly amplifying disturbances, including short- and long-time perturbations (non-modal and spectral, respectively) are identified and analyzed. Finite-amplitude transiently amplifying perturbations are shown to provide a mechanism that can bypass slower asymptotic modal linear growth and precipitate the onset of nonlinear dynamics. Direct numerical simulations are used to verify the linear analysis and track the subsequent transition of the regular capsule trains into an apparently chaotic flow. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M26.00004: Testing the paradigms of the glass transition in colloids via dynamic simulation Jialun Wang, Xiaoguang Peng, Qi Li, Gregory McKenna, Roseanna Zia Upon cooling, molecular glass-formers undergo a glass transition during which viscosity appears to diverge, and the material transitions from a liquid to an amorphous solid. However, the new state is not an equilibrium phase: material properties such as enthalpy continue to evolve in time. Rather, the material evolves toward an “intransient” state, as measured by the Kovacs signature experiments, e.g. the intrinsic isotherm, which reveals a paradoxical dependence of transition time on quench depth, and suggests that whether the glass transition occurs at the beginning or end of this transition is an open question. Colloidal glass formers provide a natural way to model such behavior, owing to the disparity in time scales that allow tracking of particle dynamics. We interrogate these ideas via dynamic simulation of a hard-sphere colloidal glassy state induced by jumps in volume fraction. We explore three methods to model the jump: evaporation, aspiration, and particle-size jumps. During and following each jump, the positions, velocities, and particle-phase stress are tracked and utilized to characterize relaxation time scales and structural changes. Analogs for the intrinsic isotherms are developed. The results provide insight into the existence of an “ideal” glass transition. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M26.00005: Dynamics of two balls in bounded shear flow of Oldroyd-B fluids Shang-Huan Chiu, Tsorng-Whay Pan, Roland Glowinski The motion of dilute sphere suspensions in bounded shear flow of Oldroyd-B fluids has been studied at zero Reynolds number. Up to the initial sphere displacement, binary encounters of spheres in bounded shear flow of Newtonian fluid are known to have either swapping or non-swapping trajectories at zero Reynolds number (Zurita-gotor et al., J. Fluid Mech. 592 (2007) 447-469). We have simulated the interaction of two spherical particles in Newtonian fluid and Oldroyd-B fluid, respectively, and compared the resulting motions of particles. The motions of two spheres in Newtonian fluid are consistent with those in literature. In Oldroyd-B fluid, swapping trajectories can be obtained for the lower values of the relaxation time. For the non-swapping cases, two spheres do not return to their original transversed position once the encounter terminates, but being closer to the mid-plane between two walls, due to the effect of the elastic force. Two spheres may also attract each other first and then form rotating dipole in bounded shear flow, depending on the value of the relaxation time and initial sphere displacement. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M26.00006: Anomalous low friction coefficient in shear thickening suspensions Cecile Clavaud, Antoine Berut, Bloen Metzger, Yoel Forterre We study the frictional behavior of classical and shear thickening suspensions under low confining pressure, by measuring the pile slope-angle in rotating drum flow experiments. We show that, at low rotation rates, the pile angle of the shear thickening suspension is about $8.5^{\circ}$, which is much lower than pile angles observed with classical suspensions in the same conditions ($\sim 25^{\circ}$). We then study the frictional behavior of silica powders in water, and show that we can switch from low to high pile angle by changing the salinity of the suspension. These results support a recent scenario for the shear-thickening transition in such non-Brownian systems, where inter particle repulsive forces suppress friction at low confining pressure. [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M26.00007: Direct numerical simulation of particle alignment in viscoelastic fluids Martien Hulsen, Nick Jaensson, Patrick Anderson Rigid particles suspended in viscoelastic fluids under shear can align in string-like structures in flow direction. To unravel this phenomenon, we present 3D direct numerical simulations of the alignment of two and three rigid, non-Brownian particles in a shear flow of a viscoelastic fluid. The equations are solved on moving, boundary-fitted meshes, which are locally refined to accurately describe the polymer stresses around and in between the particles. A small minimal gap size between the particles is introduced. The Giesekus model is used and the effect of the Weissenberg number, shear thinning and solvent viscosity is investigated. Alignment of two and three particles is observed. Morphology plots have been created for various combinations of fluid parameters. Alignment is mainly governed by the value of the elasticity parameter $S$, defined as half of the ratio between the first normal stress difference and shear stress of the suspending fluid. Alignment appears to occur above a critical value of $S$, which decreases with increasing shear thinning. This result, together with simulations of a shear-thinning Carreau fluid, leads us to the conclusion that normal stress differences are essential for particle alignment to occur, but it is also strongly promoted by shear thinning. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M26.00008: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M26.00009: Evolution of Clouds of Migrating Micron-particles with Hydrodynamic and Electrostatic Interactions. Shuiqing Li, Sheng Chen The evolution of dilute clouds of charged micron-sized particles during the migration in an external electric field is numerically investigated. The hydrodynamic interaction is modeled employing the Oseen dynamics in the limit of small-but-finite particle Reynolds number. The effects of external field and inter-particle Coulomb repulsion are accounted by a pairwise summation. As a result, with a dominant external electrostatic force, the cloud is seen to flatten into a planar configuration with particle leakage in the tail and eventually breaks up into two small clouds. Decreasing the external force or increasing the pairwise Coulomb repulsion has a similar effect on the dynamics of the cloud, i.e., decreases the scaled migrating velocity of the cloud and makes the cloud steady in its spherical shape. While this behavior bears some similarity with the transition from the Stokes regime to the micro-scale inertia dominant regime, the underlying physical mechanisms differ. Finally, the variation of the typical aspect ratio of the cloud, as a function of a scaled radial velocity of particles, is used to quantify the effect of Coulomb repulsion on the stability of the shape of the cloud. [Preview Abstract] |
Tuesday, November 22, 2016 9:57AM - 10:10AM |
M26.00010: Detecting plastic events in emulsions simulations Matteo Lulli Emulsions are complex systems which are formed by a number of non-coalescing droplets dispersed in a solvent leading to non-trivial effects in the overall flowing dynamics. Such systems possess a yield stress below which an elastic response to an external forcing occurs, while above the yield stress the system flows as a non-Newtonian fluid, i.e. the stress is not proportional to the shear. In the solid-like regime the network of the droplets interfaces stores the energy coming from the work exerted by an external forcing, which can be used to move the droplets in a non-reversible way, i.e. causing plastic events. The Kinetic-Elasto-Plastic (KEP) theory is an effective theory describing some features of the flowing regime relating the rate of plastic events to a scalar field called fluidity $f=\frac{\dot{\gamma}}{\sigma}$, i.e. the inverse of an effective viscosity. Boundary conditions have a non-trivial role not captured by the KEP description. In this contribution we will compare numerical results against experiments concerning the Poiseuille flow of emulsions in microchannels with complex boundary geometries. Using an efficient computational tool we can show non-trivial results on plastic events for different realizations of the rough boundaries. [Preview Abstract] |
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