Memory-induced alignment of colloidal dumbbells

TL;DR

This study explores how colloidal dumbbells in viscoelastic fluids exhibit complex recoil and reorientation behaviors after being driven, revealing memory effects and fluctuation phenomena relevant for asymmetric particle dynamics.

Contribution

It provides the first combined experimental and theoretical analysis of the recoil and orientation dynamics of non-spherical colloids in viscoelastic media.

Findings

Pronounced translational and angular recoil observed.

Orientation-dependent fluctuation amplification identified.

Good agreement with Langevin and microscopic models achieved.

Abstract

When a colloidal probe is forced through a viscoelastic fluid which is characterized by a long stress-relaxation time, the fluid is excited out of equilibrium. This is leading to a number of interesting effects including a non-trivial recoil of the probe when the driving force is removed. Here, we experimentally and theoretically investigate the transient recoil dynamics of non-spherical particles, i.e., colloidal dumbbells. In addition to a translational recoil of the dumbbells, we also find a pronounced angular reorientation which results from the relaxation of the surrounding fluid. Our findings are in good agreement with a Langevin description based on the symmetries of a director (dumbbell) as well as a microscopic bath-rod model. Remarkably, we find a frustrated state with amplified fluctuations when the dumbbell is oriented perpendicular to the direction of driving. Our results demonstrate the complex behavior of non-spherical objects within a relaxing environment which are of immediate interest for the motion of externally but also self-driven asymmetric objects in viscoelastic fluids.