Spontaneous vortex formation by microswimmers with retarded attractions

TL;DR

This study demonstrates that microswimmers with delayed self-propulsion can spontaneously form complex, synchronized vortex states, revealing how simple retarded interactions induce rich collective behaviors without the need for many-particle cooperation.

Contribution

The paper provides experimental evidence that retarded self-interactions in microswimmers lead to spontaneous vortex formation and critical dynamics, highlighting the role of delay-induced synchronization.

Findings

Spontaneous symmetry breaking to chiral states.

Emergence of layered orbital motion with increasing swimmers.

Synchronization driven by delay and hydrodynamic interactions.

Abstract

Collective states of inanimate particles self-assemble through physical interactions and thermal motion. Despite some phenomenological resemblance, including signatures of criticality, the autonomous dynamics that binds motile agents into flocks, herds, or swarms allows for much richer behavior. Low-dimensional models have hinted at the crucial role played in this respect by perceived information, decision-making, and feedback implying that the corresponding interactions are inevitably retarded. Here we present experiments on spherical Brownian microswimmers with delayed self-propulsion toward a spatially fixed target. We observe a spontaneous symmetry breaking to a transiently chiral dynamical state and concomitant critical behavior that does not rely on many-particle cooperativity. By comparison with the stochastic delay differential equation of motion of a single swimmer, we pinpoint the delay-induced effective synchronization of the swimmers with their own past as its key mechanism. Increasing numbers of swimmers self-organize into layers with pro- and retrograde orbital motion, synchronized and stabilized by steric and hydrodynamic interactions. Our results demonstrate how even most simple retarded non-reciprocal interactions can foster emergent complex adaptive behavior in relatively small ensembles.