Obtaining informatiion about zeta potential, electrophoretic mobility and numbers of suspended particles

An apparatus for obtaining information about the zeta potential, electrophoretic mobility and/or numbers of one or more particles 38 dispersed in a fluid comprises means 22, 24, 28 for establishing an electric field across a said dispersion of particles in a fluid to move the particles through the fluid at a velocity proportional to their electrophoretic mobilities in a direction parallel with the electric field; laser means 10 for generating a light beam of monochromatic or quasi-monochromatic electromagnetic radiation incident on the particles so that the light beam is scattered by the particles; and detector means 36 for detecting the light scattered by the particles whereby the zeta potential, electrophoretic mobility and/or numbers of the particles in the fluid may be determined, wherein the means for establishing an electric field is adapted to apply an oscillating electric field to the fluid.