Run-and-Tumble-Like Motion of Active Colloids in Viscoelastic Media

TL;DR

This paper demonstrates that active colloids in viscoelastic media can mimic run-and-tumble motion through periodic modulation of propulsion, revealing new insights into microswimmer behavior in complex fluids.

Contribution

It introduces a novel active motion mode in viscoelastic fluids where translational and orientational changes are decoupled, achieved via controlled propulsion modulation.

Findings

Enhanced translational and rotational motion at specific modulation times

Elastic stress relaxation explains the motion dynamics

Potential for new steering strategies in complex environments

Abstract

Run-and-tumble (RNT) motion is a prominent locomotion strategy employed by many living microorganisms. It is characterized by straight swimming intervals (runs), which are interrupted by sudden reorientation events (tumbles). In contrast, directional changes of synthetic microswimmers (active particles, APs) are caused by rotational diffusion, which is superimposed with their translational motion and thus leads to rather continuous and slow particle reorientations. Here we demonstrate that active particles can also perform a swimming motion where translational and orientational changes are disentangled, similar to RNT. In our system, such motion is realized by a viscoelastic solvent and a periodic modulation of the self-propulsion velocity. Experimentally, this is achieved using light-activated Janus colloids, which are illuminated by a time-dependent laser field. We observe a strong enhancement of the effective translational and rotational motion when the modulation time is comparable to the relaxation time of the viscoelastic fluid. Our findings are explained by the relaxation of the elastic stress, which builds up during the self-propulsion, and is suddenly released when the activity is turned off. In addition to a better understanding of active motion in viscoelastic surroundings, our results may suggest novel steering strategies for synthetic microswimmers in complex environments.