Electrophoresis is an exceptionally effective method for separating small particles or large molecules in the colloidal size range, and is widely used in biological and clinical studies to separate cells, viruses, and large proteins. It has become the standard analytical tool for detecting the presence of these materials. Despite the success of electrophoresis in analytical and research applications, there has been much less success in using electrophoresis in preparative-scale applications to separate large quantities of materials. A new or modified concept is described that eliminates some of the most serious problems that have arisen in adopting electrophoresis for preparative-scale separations. The new concept uses a narrow-gap flow system between the electrodes and eliminates or greatly reduces problems with thermal convection. In addition, the narrow gap can be a convenient annular region between two cylindrical electrodes. The electric potential is applied across the annulus (or across the narrow flow channel) and the potential applied to the electrodes is reversed periodically. Between the reversals of the electric field, the inner electrode (or one side of the narrow flow gap) is rotated periodically in different directions. This periodic motion and alternating of the applied field displaces charged particles in the angular direction. The rate of displacement depends on the electrophoretic mobility of the particles.