DIRECTIONAL MOTION OF BROWNIAN PARTICLES INDUCED BY A PERIODIC ASYMMETRIC POTENTIAL

STRUCTURES possessing spatial asymmetry should act as pumps in the pre sence of dissipation alone(1-4), without the need for macroscopic forc es or temperature differences(5) to drive vectorial motion. It has bee n shown theoretically(2-4,6,7) that particles subjected to an asymmetr ic periodic potential can display net directional motion even if the s pace-averaged forte is zero. Here we demonstrate such behaviour experi mentally. We have studied the behaviour of colloidal particles suspend ed in solution and exposed to a sawtooth dielectric potential which is turned on and off periodically. The particles exhibit net motion with a velocity that depends on their size, suggesting applications in sep aration processes for objects in the size range 0.1-5 mu m-a range tha t includes biological structures such as viruses, cells and chromosome s(8). We furthermore point out the analogy between our device and moto r protein assemblies.