Memory effect by coupling between translational and rotational Brownian motion in water-ethanol mixtures

Abstract

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. Our findings reveal that the anomalous Brownian motion can be attributed to the memory effect arising from the coupling between rotational and translational motion. It was deduced that the relaxation time of sub-microsecond rotational motion serves as a memory effect, influencing translational motion and leading to anomalously large Brownian displacements over a timescale of seconds. This study demonstrates that short-time memory effects can be effectively investigated through long-duration measurements.