Tracking a colloidal particle for the measurement of dynamic entropies

The paper describes the apparatus and measurement procedure for tracking long time trajectories of a single colloidal particle undergoing Brownian motion in water. The bulk motion obeys the Einstein–Stokes formulation, while sedimented motion is slowed by an additional drag known as the wall-drag effect. The sedimented particle trajectory is long enough to show diffusive motion spanning four decades in time. We describe the sample preparation, tracking routine, data-acquisition and noise. The experiment shows that Brownian motion exhibits a positive dynamic entropy consistent with microscopic chaos. The theoretical relevance of the experimental results is discussed.