The fraction of foulant particles deposited on Anopore aluminum oxide rnicrofiltration membranes during tangential flow filtration was measured by direct visual observation (DVO). Yeast cells and 4.5 mu m, 10 mu m, and 20 mu m latex particles were used as model foulants. The fractional particle deposition was found to increase with increasing permeate flux and decrease with increasing wall shear rate. At a fixed permeate flux and wall shear rate, the fractional deposition decreased with increasing particle size, allowing for particle segregation by size. For feed suspensions at the isoelectric point of the membrane (pH 8), individual particles and cells were frequently observed rolling over the membrane surface. To better understand the observed fractional deposition and foulant rolling speed, a mathematical model of a sphere rolling over a rough, permeable surface due to viscous shear flow was developed. The primary parameter in the model is the dimensionless permeate flux, defined as the ratio of permeate flux (velocity of fluid flowing normal to the surface) to the characteristic shear velocity (velocity of fluid flowing tangentially over the surface at one particle radius above the surface). For a given permeate flux, a sphere will stick to the surface only when it encounters a roughness element taller than a critical height. The time-averaged translational velocities of particles rolling over the membrane surface were also calculated using the model and are in good agreement with experimental results. with values reduced by interactions with the membrane to typically 20-50% of the undisturbed tangential fluid velocity at one radius above the membrane. (c) 2005 Elsevier B.V. All rights reserved.