For micro-machined membranes, it is possible to choose pore size, pore geometry and membrane porosity, within certain limits. Different pore geometries (circular, square, slit shaped and triangular pores), particle size to pore size ratios, pore edges and membrane porosities were evaluated with lattice-Boltzmann computer simulations and torque balance considerations for various modes of operation. We focused on hydrodynamic interactions and assumed uncharged neutral surfaces of the particle and the pore. However, the model can easily be extended with additional relations for such interactions in practical systems with defined properties. It was concluded that pore geometry can have a large effect on the flux (up to 60%). Further, the effect of shielding could be quantified. Above a surface coverage of 0.05, the particles effectively shield each other from the flow field, therewith necessitating either a higher cross flow velocity or a lower transmembrane pressure for particle removal. Based on the simulations, an extended criterion for the critical flux was developed, which includes the effects of pore geometry, particle to pore size ratio and membrane porosity. Different optimal membrane choices follow for processes aimed at retention of all particles, and for processes aimed at fractionation of particles into different fractions. (c) 2005 Elsevier B.V. All rights reserved.