Faxen's laws of a composite sphere under creeping flow conditions

Under creeping flow conditions, Faxen's laws are derived for a composite sp here comprising a solid core covered by a permeable layer of arbitrary thic kness, The derivations are carried out by applying reciprocal theorem in co mbination with fluid velocity and pressure distributions in certain simple how as a comparison held. In this regard, the fluid velocity disturbances c aused by a composite sphere subject to a simple shear how and a rotational how are solved individually. In the limiting case where the solid core vani shes, the resulting Faxen expressions for the drag force, torque, and stres slet compare very well with the existing Faxen's law for a porous sphere. I t is found that when the porous layer is thick enough and its permeability is sufficiently low, the hydrodynamic behavior of a composite sphere can be approximated by that of a porous particle with equal permeability. This ca n be explained by the fact that the fluid cannot penetrate deeply into a po rous layer of low permeability to flow through the pores near the core surf ace, and thereby the fluid can hardly feel the resistance from the core sur face. (C) 2000 Academic Press.