Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G25: Suspensions II |
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Chair: Ubaldo Cordova-Figueroa, University of Puerto Rico-Mayagüez Room: 328 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G25.00001: Osmotic motor under fixed flux conditions Ubaldo Cordova-Figueroa, John Brady, Sergey Shklyaev We consider an osmotic motor that emits product particles from half of its surface. This asymmetry produces a difference of the product concentration, and hence of the osmotic pressure, between the active and passive surfaces, resulting in motion of the motor. Hydrodynamic interactions between the motor and the small product particles are included in the analysis following the theory developed by Brady (JFM, 2011). Limiting cases of slow and fast motion of the motor are considered analytically; otherwise the velocity of self-propulsion is determined numerically. Brownian Dynamic simulations are in a good agreement with the theoretical predictions. The analysis is also generalized to a reactive patch, which is smaller or larger than a hemisphere; in either case this diminishes the motor velocity. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G25.00002: Chemical sailing: Nonspherical catalytic motors Sergey Shklyaev, John Brady, Ubaldo Cordova-Figueroa Self-propulsion of a chemically active particle (e.g. an osmotic motor) is a promising area that has a number of far- reaching applications. The conventional strategy is to construct a Janus particle that is chemically active on a portion of its surface, whereby an asymmetric concentration gradient of reactant is generated leading to osmotic propulsion. However, a nonspherical motor with uniform intensity of chemical reaction on its surface can also lead to osmotic propulsion, which allows for the design of simpler, cheaper motors. Wei \& Jan (JFM, 2010) considered a weakly nonspherical motor and found the velocity of self-propulsion to be linear in terms of nonsphericity. However, we show that their calculations are in error and that net propulsion occurs at second order with respect to the nonsphericity. Moreover, hydrodynamic interactions (HI), which were neglected by Wei \& Jan, can be of crucial importance---even the direction of self- propulsion can change with HI. Large departures from sphericity are also investigated numerically in an effort to determine the optimum shape for maximum propulsion. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G25.00003: A liquid bridge connecting moving porous surfaces Morteza Gharib, Amir Gat, Homayun Navaz We study the coupled problem of a liquid bridge connecting two porous surfaces where the gap between the surfaces is an externally controlled function of time. The relative motion between the surfaces affects the geometry and the pressure distribution of the liquid bridge, thus influencing the diffusion speed and penetration topology within the porous material. Utilizing the lubrication approximation and Darcy's phenomenological law we obtain a relation between the diffusion into the porous surface and the relative motion between the surfaces. A scheme to control the diffusion topology is presented and illustrated for the case of conical penetration topology with an arbitrary cone opening angle. Analytic expressions describing the penetration topology for the case of constant speed of the surfaces and the relative motion between the surfaces required to create a conical penetration topology are obtained and compared to experimental and numerical data. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G25.00004: Hydrodynamic interactions between two vesicles Pierre-Yves Gires A giant vesicle is a closed elastic membrane containing a liquid, inside another liquid. Its size is around 10 microns. If a suspension of such objects is sheared, they sometimes come close and interact hydrodynamically. We studied how these interactions affect the trajectories of the vesicles. For this, we model the properties of the membrane, assuming that the area of a surface element is constant in the course of time, and that it resists bending. We also assume that the inside and outside fluids are Newtonian, and are in the creeping regime. To solve the partial differential equations arising from this model, we used two methods : an asymptotic expansion around spherical shapes for vesicles far away from each other (3d case), and a boudary integral method (2d case). We find that vesicles repel, and that this repulsion decreases with initial transverse distance. We compare our results with experimental results performed with vesicles flowing in microfluidic devices. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G25.00005: Convective Polymer Depletion on Pair Particle Interactions Tai-Hsi Fan, Takashi Taniguchi, Remco Tuinier Understanding transport, reaction, aggregation, and viscoelastic properties of colloid-polymer mixture is of great importance in food, biomedical, and pharmaceutical sciences. In non-adsorbing polymer solutions, colloidal particles tend to aggregate due to the depletion-induced osmotic or entropic force. Our early development for the relative mobility of pair particles assumed that polymer reorganization around the particles is much faster than particle's diffusive time, so that the coupling of diffusive and convective effects can be neglected. Here we present a nonequilibrium two-fluid (polymer and solvent) model to resolve the convective depletion effect. The theoretical framework is based on ground state approximation and accounts for the coupling of fluid flow and polymer transport to better describe pair particle interactions. The momentum and polymer transport, chemical potential, and local viscosity and osmotic pressure are simultaneously solved by numerical approximation. This investigation is essential for predicting the demixing kinetics in the pairwise regime for colloid-polymer mixtures. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G25.00006: Non-equilibrium depletion interactions: dual-probe microrheology Roseanna Zia, John Brady Non-equilibrium depletion interactions in colloidal dispersions are studied via nonlinear, dual-probe microrheology, theoretically and via Brownian dynamics simulation. We study the interactive force between a pair of probe particles translating with equal velocity through a colloidal dispersion with their line of centers transverse to the external forcing. The character of the microstructure surrounding the probes is determined by the distance $R$ by which the two probes are separated and by the strength of the external forcing compared to the thermal force of the bath, which defines a P\'{e}clet number, $Pe=Ua/D_b$, where $U$ is the probe velocity, $a$ is its size and $D_b$ the diffusivity of the bath particles. Osmotic pressure gradients develop as the microstructure is deformed, giving rise to an interactive force between the probes. This force is studied for a range of $Pe$ and $R$. For all separations $R>2a$, the probes attract when $Pe$ is small. As the strength of the forcing increases, a qualitative change in the interactive force occurs: the probes repel each other. The separation $R$ at which the attraction-to-repulsion transition occurs decreases as $Pe$ increases as the entropic depletion attraction becomes weak compared to the force-induced osmotic repulsion. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G25.00007: Lattice Boltzmann Method For Structured Flexible Bodies in Fluid Flows D. Qi, T. Wu, R. Guo A new simulation method for a structured flexible solid particles is developed. In this method, a spring lattice model is generalized to include two- and three-body force interactions and construct a structured fiber so that the fiber could be freely deformed in any irregular shapes. Lattice Boltzmann method and Drace Delta function are employed to deal with complex interaction in the interfaces between fluid and solid particles. In this new method, constrain forces are added into the fluid particles within the solid regime so that the inside fluid particles have the same property as the solid. Further, rotation and motion of a flexible fiber in a shearing flow and in a sediment flow, respectively, are numerically simulated. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G25.00008: The effect of dissolved oxygen on electrohydrodynamic aggregation of colloidal particles near electrodes T.J. Woehl, N.D. Browning, W.D. Ristenpart Colloidal particles suspended in dilute electrolytes have been widely observed to aggregate laterally along electrodes in response to oscillatory electric fields. Studies on the effects of particle type and electrode material have suggested that electrochemical reactions play a role in driving aggregation, but the detailed mechanism remains unclear. Here we demonstrate that, at sufficiently low frequencies, the dissolved oxygen (DO) content of the suspending electrolyte strongly affects the aggregation behavior of micron-scale particles. At 100 Hz, colloids in aqueous KCl saturated with oxygen exhibit an aggregation rate three times larger compared to aggregation in deoxygenated solutions. In contrast, no effect of DO content is observed at 500 Hz with KCl, and no effect is observed at either frequency when the suspending electrolyte is KOH. We investigate the role of DO using cyclic voltammetry, and we interpret the observations in terms of the effect of DO on the magnitude of the electrochemical current driving the electrohydrodynamic flow. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G25.00009: Evolution of streamers in sedimentation of fibre suspensions bounded by vertical walls Feng Zhang, Anders Dahlkild, Fredrik Lundell The simulation, based on the Navier-Stokes equations coupled to a transport equation for the PDF of fibres, shows that a series of alternating structures of streamers and backflow regions emerge continuously from the walls until they converge in the midst of the domain. For moderate times, this agrees qualitatively with experimental and theoretical literature. In experimental literature, the evolution of the flow structure is restricted in time due to the finite height of the vessel and a steady state is not reached. However, our simulation in a vessel of infinite height obtained an increasing wavelength evolution due to the congregation of the streamers. In the end, there is constantly only one streamer left, and it drifts randomly to one side of the container until the evolution reaches a steady state. It also shows that the maximum number of streamers increases with increasing values of $Re$, fibres concentration, fibres aspect ratio, and container width. [Preview Abstract] |
Monday, November 21, 2011 9:57AM - 10:10AM |
G25.00010: Rheology of a dense capsule suspension R. Murthy Kalluri, Prosenjit Bagchi The rheology of a dense suspension of deformable capsules in a linear shear flow is studied at low Reynolds numbers. Three- dimensional numerical simulations are conducted for initially spherical capsules using a front-tracking method. Capsules are represented as drops of Newtonian fluid enclosed by an elastic membrane and suspended in another Newtonian liquid of different viscosity. The capsule volume fraction ranges up to about 26\%. This study is motivated by our earlier works [Bagchi \& Kalluri, Phys Rev E, \textbf{81}, 056320 (2010); Bagchi \& Kalluri, J Fluid Mech, \textbf{669}, 498 (2011)] where we show that for a {\it dilute} suspension the shear viscosity exhibits a nonmonotonic trend with respect to the internal-to-external fluid viscosity ratio; in particular, the shear viscosity exhibits a minimum at an intermediate viscosity ratio. In case of a {\it dense} suspension, we find that the shear viscosity minimum gradually diminishes as with increasing capsule volume fraction. We explain this result by decomposing the particle shear stress into elastic and viscous components. The elastic component is observed to increase but the viscous component remains constant with respect to increasing volume fraction. It is also shown that the elastic contribution is shear-thinning, but the viscous contribution is shear-thickening. [Preview Abstract] |
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