Sorting of Brownian particles by the pulsed application of an asymmetric potential

We submit a suspension of Brownian latex spheres in water to a spatially periodic and asymmetric potential successively switched on and off. Such a potential is created by dielectrophoretically induced forces resulting from the application of a high-frequency electric field between a plane electrode and a blazed optical grating. As expected, we observe a net macroscopic drift of the particles. Their average velocities have been measured as a function of the time during which the field is switched off. We observe a quantitative agreement with the theoretical predictions. In particular, the velocities strongly differ according to the size of the latex spheres, which should open the way to devices aimed at the separation of micrometer and submicrometer particles. The geometry of the gratings as well as the concentration and the surface functions of the particles has also been investigated.