Tracking a colloidal particle for the measurement of dynamic entropies

The paper describes the apparatus and measurement procedure for tracking lo ng time trajectories of a single colloidal particle undergoing Brownian mot ion in water. The bulk motion obeys the Einstein-Stokes formulation, while sedimented motion is slowed by an additional drag known as the wall-drag ef fect. The sedimented particle trajectory is long enough to show diffusive m otion spanning four decades in time. We describe the sample preparation, tr acking routine, data-acquisition and noise. The experiment shows that Brown ian motion exhibits a positive dynamic entropy consistent with microscopic chaos. The theoretical relevance of the experimental results is discussed. (C) 2001 Published by Elsevier Science B.V.