Microstructure from simulated Brownian suspension flows at large shear rate

Pair microstructure of concentrated Brownian suspensions in simple-shear flow is studied by sampling of configurations from dynamic simulations by the Stokesian Dynamics technique. Simulated motions are three dimensional with periodic boundary conditions to mimic an infinitely extended suspension. Hydrodynamic interactions through Newtonian fluid and Brownian motion are the only physical influences upon the motion of the monodisperse hard-sphere particles. The dimensionless parameters characterizing the suspension are the particle volume fraction and Peclet number, defined, respectively, as phi=(4pi/3)na(3) with n the number density and a the sphere radius, and Pe=6pieta(gamma) over dot a(3)/kT with eta the fluid viscosity, (gamma) over dot the shear rate, and kT the thermal energy. The majority of the results reported are from simulations at Pe=1000; results of simulations at Pe=1, 25, and 100 are also reported for phi=0.3 and phi=0.45. The pair structure is characterized by the pair distribution function, g(r)=P-1\1(r)/n, where P-1\1(r) is the conditional probability of finding a pair at a separation vector r. The structure under strong shearing exhibits an accumulation of pair probability at contact, and angular distortion (from spherical symmetry at Pe=0), with both effects increasing with Pe. Flow simulations were performed at Pe=1000 for eight volume fractions in the range 0.2less than or equal tophiless than or equal to0.585. For phi=0.2-0.3, the pair structure at contact, g(\r\=2)equivalent tog(2), is found to exhibit a single region of strong correlation, g(2)>>1, at points around the axis of compression, with a particle-deficient wake in the extensional zones. A qualitative change in microstructure is observed between phi=0.3 and phi=0.37. For phigreater than or equal to0.37, the maximum g(2) lies at points in the shear plane nearly on the x axis of the bulk simple shear flow U-x=(gamma) over dot y, while at smaller phi, the maximum g(2) lies near the compressional axis; long-range string ordering is not observed. For phi=0.3 and phi=0.45, g(2)similar toPe(0.7) for 1less than or equal toPeless than or equal to1000, a slower increase than the g(2)similar toPe predicted theoretically for phi<<1 [Brady and Morris, J. Fluid Mech. 348, 143 (1997)]. Spherical harmonic decomposition of g(r) was performed, and for Pe=1000, expansion convergence is found to be nearly complete when harmonics Y-lm to the level l=10 are included. (C) 2002 American Institute of Physics.