Memory Effect by Coupling Between Translational and Rotational Brownian Motion in Water–Ethanol Mixtures

The Brownian motion in water–ethanol mixtures exhibits abnormally large displacements. Using falling-ball viscometry applied to colloidal particles, we experimentally verified that no anomaly exists in the viscosity coefficient of the solution. We concluded that the anomalous Brownian motion displacement is due to anomalous thermal fluctuations, not viscous forces. We proposed that the coupling of rotational and translational Brownian motion may result in anomalous thermal fluctuations. The viscosity fluctuation converts rotational motion into translational motion, and the displacement increases by that amount. The anomalous thermal disturbance continues for the relaxation time of rotational Brownian motion, which produces a type of memory effect.