Memory-induced transition from a persistent random walk to circular motion for achiral microswimmers

TL;DR

This study reveals that achiral microswimmers in viscoelastic fluids transition from random to circular motion due to memory effects, with their dynamics controllable by propulsion speed, challenging previous assumptions about symmetry and motion.

Contribution

The paper demonstrates experimentally and theoretically that memory effects in viscoelastic fluids induce a transition to circular motion in achiral microswimmers, a phenomenon not observed in Newtonian fluids.

Findings

Achiral microswimmers exhibit a transition to circular motion above a critical speed.

Circular orbits can spontaneously reverse direction and depend non-linearly on propulsion speed.

Memory effects in viscoelastic fluids cause non-Markovian dynamics leading to this transition.

Abstract

We experimentally study the motion of light-activated colloidal microswimmers in a viscoelastic fluid. We find that, in such a non-Newtonian environment, the active colloids undergo an unexpected transition from enhanced angular diffusion to persistent rotational motion above a critical propulsion speed, despite their spherical shape and stiffness. We observe that, in contrast to chiral asymmetric microswimmers, the resulting circular orbits can spontaneously reverse their sense of rotation and exhibit an angular velocity and a radius of curvature that non-linearly depend on the propulsion speed. By means of a minimal non-Markovian Langevin model for active Brownian motion, we show that these non-equilibrium effects emerge from the delayed response of the fluid with respect to the self-propulsion of the particle without counterpart in Newtonian fluids.