Microscopic motion of particles flowing through a porous medium

Stokesian dynamics simulations are used to study the microscopic motion of particles suspended in fluids passing through porous media. Model porous me dia with fixed spherical particles are constructed, and mobile ones move th rough this fixed bed under the action of an ambient velocity field. The por e scale motion of individual suspended particles at pore junctions are firs t considered. The relative particle flux into different possible directions exiting from a single pore, for two- and three-dimensional model porous me dia is found to approximately equal the corresponding fractional channel wi dth or area. Next the waiting time distribution for particles which are del ayed in a junction due to a stagnation point caused by a flow bifurcation i s considered. The waiting times are found to be controlled by two-particle interactions, and the distributions take the same form in model porous medi a as in two-particle systems. A simple theoretical estimate of the waiting time is consistent with the simulations. It is found that perturbing such a slow-moving particle by another nearby one leads to rather complicated beh avior. Finally, the stability of geometrically trapped particles is studied . For simple model traps, it is found that particles passing nearby can ''r elaunch'' the trapped particle through its hydrodynamic interaction, althou gh the conditions for relaunching depend sensitively on the details of the trap and its surroundings. (C) 1999 American Institute of Physics. [S1070-6 631(99)00801-6].