This invention relates to a method and device for separating particles according to their diffusivities in a separation medium by means of a spatially and temporally varying electric potential. The method takes advantage of the transport of charged particles subject to an electric potential that is cycled between an off-state (in which the potential is flat) and one or more on-states, in which the potential is preferably spatially periodic with a plurality of eccentrically shaped stationary wells. The potential wells are at a constant spatial positions in the on-state. Differences in liquid-phase diffusivities lead to charged particle separation. A separation medium fills physically defined separation lanes (15) in the device. Electrodes deposited substantially transverse to the lanes create the required potential. Advantageously, injection ports (16) allow sample loading, and special gating electrodes focus the sample prior to separation.