In order to separate, within a fluid (1), certain particles (2) contained therein, this fluid is arranged in a corridor (C) partly defined by two faces which are close together and substantially parallel to each other and to the direction of separation E and an exciting field is simultaneously applied to the entire volume of fluid contained in this corridor, according to a direction having at least one component perpendicular to the direction E, which exciting field varies along the said direction in a curve consisting of a regular sequence of mutually identical asymmetric patterns, the mean of this field, taken at each instant along the direction E, being zero, and means are provided for repetitively varying the effect of the exciting field on the particles. The substantially parallel faces may be electrodes between which a potential difference source is connected for producing an electric field thereby exciting particles in the corridor (C). At least one of the electrodes has a corrugated surface with a sequence of asymmetric grooves transverse to the separation direction.