Abstract: Detail is apparatus for collecting particles from a liquid, including a first electrode having an upper surface defining one or more substantially circular areas of a predetermined diameter separated from a counter electrode, having an uppersurface bounding at least in part the circular area or areas of the first electrode, by a predetermined gap and equipment for applying an alternating potential difference of a predetermined magnitude at a predetermined frequency lower than the charge relaxation frequency of the liquid across the electrodes whereby particles from the liquid are focused onto the circular area or areas of the first elect through an induced electro-osmotic flow in the liquid.