We have experimentally applied some concepts of ''force-free'' motion
to micron size particles (latex beads). The coupling of dissipation an
d local spatial asymmetry of the potential experienced by the beads ca
n put them into motion. The potentials used in these experiments are o
f dielectrophoretic nature. To that end, electrodes of particular shap
es were used in order to submit the considered suspensions to inhomoge
neous ac electric fields. Two regimes were explored: i-the Brownian ra
tchet case in which a Brownian particle is successively trapped in a f
actory roof-like potential and left ii-ee to diffuse. ii-the shifted r
atchets case in which two potentials exhibiting similar characteristic
s are applied successively, one of them being shifted by a fraction of
their common period relatively to the other. In both cases, a good ag
reement with the theoretical predictions was observed. In particular,
particles of different sizes were characterized by different macroscop
ic velocities leading to the prospect of promising separation techniqu
es. (C) 1998 American Institute of Physics.