Measuring the electrophoretic mobility in semi-diluted suspensions by dynamic light scattering

Homodyne detection of light scattered by a colloidal suspension is a well known method of determining the size of the colloidal particles. Heterodyne detection is achieved when the scattered beam interferes with the incident beam on the detector. Heterodyne detection can be used to measure the average velocity of the particles in the suspension. In this paper we present measurements of the electrophoretic mobility of colloidal particles using heterodyne detection of light scattering. The measurements have been done up to concentrations of 3.5% of silica particles with a diameter of the order of 100 nm, in a mixture of 50% toluene and 50% ethanol. In order to make possible the measurements at these concentrations the liquid mixture is chosen as to match the index of refraction of the particles, thus resulting in a transparent suspension.