Boundary effect on slow motion of a composite sphere perpendicular to two parallel impermeable plates

The boundary effect on the perpendicular motion of a composite sphere towar ds a plane wall or between two parallel plates is investigated under the cr eeping flow conditions. The composite particle consists of a solid core and a porous shell. No restriction is placed on the shell thickness relative t o the core size. The fluid flow inside the porous layer is governed by the Brinkman equation. A boundary collocation method is used to analyze the gen eral case where the shell thickness and separation distance between the par ticle and the wall can be arbitrary. A lubrication theory is also employed to examine the special case of a particle with a thin permeable layer in ne ar contact with a single plane. A good agreement between the results from b oth methods is attained. It is found that the hydrodynamic effect of the bo undaries on the drag force experienced by a composite sphere or a porous on e is weaker than that by a solid particle. While the drag force of a porous particle having a low to moderate permeability is a monotonic, decreasing function of the separation distance, a weak maximum drag may occur for a sp here with a very high permeability at a certain distance from the wall. Thi s behavior agrees qualitatively with what Payatakes and Dassios (1987, Chem ical Engineering Communications, 58, 119-138) and Burganos, Michalopoulou, Dassios and Payatakes (1992, Chemical Engineering Science, 117, 85-88) disc overed using Darcy's law. (C) 2000 Elsevier Science Ltd. All rights reserve d.