Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session PV: Suspensions II |
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Chair: Jerzy Blawzdziewicz, Yale University Room: Ballroom B |
Tuesday, November 25, 2008 11:35AM - 11:48AM |
PV.00001: Force induced microdiffusivity of colloidal particles Roseanna Zia, John Brady \newcommand{\te}[1]{\mbox{\boldmath$ #1 $}} In constant force microrheology the velocity of the probe particle fluctuates owing to interactions with the surrounding medium. On long time scales, this fluctuating velocity gives rise to a diffusive motion of the probe particle. We study this diffusive motion as the Peclet number, $Pe$ -- the ratio of the strength of the external driving force, $\te{F}^{ext}$, compared to thermal forces, $kT/a$ -- is varied. Here, $kT$ is the thermal energy and $a$ the probe size. At small $Pe$, Brownian motion dominates and the diffusive behavior characteristic of passive microrheology is recovered. At the other extreme of high Peclet numbers the motion is still diffusive, and the diffusivity becomes ``force-induced'' scaling as $\te{F}^{ext}/\eta$, where $\eta$ is the viscosity of the solvent. Specific calculations are performed for a probe particle of size $a$ immersed in a background of colloidal bath particles of size $b$. The diffusive motion becomes increasingly anisotropic as the Peclet number is increased -- motion parallel to the direction of forcing exceeding that transverse. The ``force-induced'' microdiffusivity is compared with the analogous ``shear-induced'' diffusivity found in macrorheological measurements. [Preview Abstract] |
Tuesday, November 25, 2008 11:48AM - 12:01PM |
PV.00002: Self-organization of fibers in a flow between two counter-rotating discs Charlotte Ahlberg, Fredrik Lundell, Daniel S\"oderberg The behavior of fibers suspended in a flow between two flat counter-rotating discs has been studied experimentally. A CCD-camera was used to capture images of the fibers in the flow. Image analysis based on the concept of steerable filters extracted the position and orientation of the fibers in the plane of the discs. Experiments were performed for gaps between the discs of 0.2 to 0.9 fiber lengths, and for equal absolute values of the velocities for the upper and lower disc. The length-to-diameter ratio of the fibers was 23. Depending on the angular velocities of the discs and the gap between them, the fibers were found to organize themselves in fiber trains. A fiber train is a set of fibers positioned one after another in the tangential direction with a close to constant fiber-to-fiber distance. Each individual fiber is aligned in the radial direction (i.e. normal to the main direction of the train). The experiments show that the number of fibers in a train increases when the gap between the discs decreases. Furthermore, the number of fibers in a train decreases at both high and low angular velocities with an optimum in between. [Preview Abstract] |
Tuesday, November 25, 2008 12:01PM - 12:14PM |
PV.00003: High Gradient AC Dielectrophoretic Filtration Boris Khusid, Yueyang Shen Dielectrophoresis is the motion of an object under forces resulting from electric field gradients. Unlike a mechanical filter, particles flowing through a dielectrophoretic filter are attracted towards the electrodes and captured in the filter by the dielectrophoretic force, even though their average size can be substantially smaller than the filter pore size. We report a new, economic, easily scaled up method for the fabrication of dielectrophoretic filters. The concept utilizes winding of metal and plastic meshes. In this design, two metal meshes serving as energized and grounded electrodes are mechanically and electrically separated with a plastic mesh. The proposed technology d allows for a reduction in the applied AC voltage and electric power by employing fine mesh materials. The particle captivity of an AC dielectrophoretic filter is governed by the mesh size, the particle size and polarizability, the flow rate, the field frequency, and the peak-to-peak voltage. [Preview Abstract] |
Tuesday, November 25, 2008 12:14PM - 12:27PM |
PV.00004: Effect of hydrodynamic forces on particle velocity fluctuations in suspensions at moderate Reynolds number Sudheer Tenneti, Rahul Garg, Rodney Fox, Shankar Subramaniam Direct numerical simulations (DNS) of monodisperse suspensions with high particle inertia and moderate fluid inertia are performed using an immersed boundary method (IBM) to quantify the effect of hydrodynamic forces on particle velocity fluctuations. The evolution of the second moment of particle velocity fluctuations is driven by the correlation between fluctuating particle acceleration and fluctuating particle velocity. This correlation arises in part due to hydrodynamic interactions with neighboring particles, and it is not satisfactorily predicted by existing drag laws for the particle acceleration used in conjunction with the particle velocity distribution. A new Langevin model for the fluctuating particle acceleration is proposed, which yields promising results when compared with the DNS data. The source and sink terms in the particle velocity second moment equation that arise due to hydrodynamic interactions are quantified using the DNS data. [Preview Abstract] |
Tuesday, November 25, 2008 12:27PM - 12:40PM |
PV.00005: Filtration of a fiber suspension: velocity measurements and Fokker-Planck orientatation simulations Gabriele Bellani, Fredrik Lundell, L. Daniel S\"oderberg We present an experimental and numerical study on the filtration of a dilute suspension of flexible fibers. The experiments were performed in a vertical channel, and sedimentation occurred in the direction of the filtration flow. The channel had a square cross section. The suspension was made optically transparent by matching the index of refraction of fluid (mixture of glycerine) and fibers (fluorocarbon, d=260 $\mu$m, l=9.8, 18.6 mm), and Particle Image Velocimetry was used to measure the time-resolved velocity field of the fluid phase in the proximity of a permeable screen, where the fibers were retained. The velocity was measured in a vertical plane located in a region where wall-effects are negligible. The evolution of the orientation distribution of the fibers in this plane was calculated with the Fokker-Planck equation, based on the measured flow fields. This was done under the assumption of creeping flow. Both filtration velocity and fiber aspect ratio have a considerable effect on the final orientation distribution of the network formed on the wire. Our methodology allows us to distinguish between these two factors. [Preview Abstract] |
Tuesday, November 25, 2008 12:40PM - 12:53PM |
PV.00006: Hydrodynamic Interactions Mediated by Polymer Depletion Effect Tai-Hsi Fan, Bin Xie Polymer depletion has significant impact on the transport and binding of proteins as well as the aggregation of macromolecules in a molecularly crowded environment. In many synthetic colloid-polymer mixtures, polymer depletion can be used to control the stability and dynamics of colloidal dispersions. For nonadsorbing polymer solutions in which depletion phenomenon occurs, polymer chains tend to move away from the region surrounding the suspended particles to avoid the loss of configuration entropy. This depletion zone complicates particle's diffusion behavior and may cause depletion-induced flocculation due to the unbalanced osmotic force. The thermodynamic origin of the depletion-induced entropy force is well understood, but the hydrodynamics involved in particle-particle interactions taking into account the depletion effect has not been previously studied. We analyze the hydrodynamic mobility of a pair of interacting Brownian particles mediated by the polymer depletion effect. Analysis will be presented specially for limiting cases when depletion zones of uncharged particles overlap. The proposed theoretical model is important for predicting the aggregation kinetics of nucleation- and diffusion-limited flocculation processes. [Preview Abstract] |
Tuesday, November 25, 2008 12:53PM - 1:06PM |
PV.00007: Coating with colloids by receding contact line Guillaume Berteloot, Limat Laurent, Lequeux Francois, Chi-Tuong Pham, Adrian Daerr, Mathieu Receveur Many coating processes use evaporation. But such coatings are usually inhomogeneous because of the evaporation singularity at the contact line. We are thus investigating the effect of this singularity on dip-coating. In dip-coating, two flows are in competition: one inwards due to the receding contact line, the other outwards due to evaporation, and the equiibrium of thes flows predicts the thicknes of the deposit. There are two dip-coating regimes: one controlled by evaporation, and the known Landau-Levich regime. A minimum deposit thickness is expected between these two regimes. Using different microscopy techniques, we found out that there was a minimum in the deposit thickness, but that the actual mesoscopic order strongly varies depending on the contact line velocity. In the stick-slip regime, we can also link the spatial frequency of the stick-slip motion with the contact line velocity. Eventually, the thinnest deposits exhibits iridescence, which means that we are close to a photonic cristal structure. [Preview Abstract] |
Tuesday, November 25, 2008 1:06PM - 1:19PM |
PV.00008: Expansion flows in suspensions Manuj Swaroop, John Brady Suspension flows can lead to variations in particle volume fraction, thus making the particle phase compressible on a macroscopic scale. The stress in such a flow is characterized by an effective bulk viscosity ($\kappa_{eff}$) in addition to the effective shear viscosity of the suspension. The bulk viscosity of a suspension of particles relates the deviation of the trace of the macroscopic or averaged stress from its equilibrium value to the average rate of expansion. The equilibrium stress is the sum of the fluid pressure and the osmotic pressure of the suspended particles. Variations in particle volume fraction are modeled by having a compressible fluid expand uniformly at a constant rate, causing the particles suspended in it to move apart. The rigid particles cannot expand, and create a disturbance flow that contributes to the total mechanical pressure in the system, thereby changing the effective bulk viscosity. Explicit formulae have been derived to compute the bulk viscosity for all volume fractions of suspended rigid particles and for all expansion rates. The hydrodynamic forces between particles including the strong lubrication interactions near contact play an important role at high concentrations. The bulk viscosity of concentrated suspensions with full hydrodynamic interactions is determined via direct simulation by adapting the Stokesian Dynamics paradigm to allow for a uniform rate of expansion. [Preview Abstract] |
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