Periodic forcing of a Brownian particle

We study the effect on a Brownian particle (2 \ensuremath{\mu}m diameter polystyrene sphere in water) of an infrared optical tweezer moving in a circle. For a given potential depth of the optical trap, three different regimes for the particle motion are observed as a function of the trap velocity. For small velocity of the tweezer (typically 100 \ensuremath{\mu}m/s), the particle is trapped and moves with the beam. For intermediate velocities (between 100 \ensuremath{\mu}m/s and 3 mm/s), the particle escapes but is caught by the returning trap: its mean angular velocity scales asymptotically as the inverse of the trap rotation frequency. For large tweezer velocities (g3 mm/s), the particle diffuses along the circle but is confined in the radial direction. We describe these observations by a simple deterministic model. We justify the use of this model solving the corresponding Fokker-Planck equation.