Universal Trajectories of Motile Particles Driven by Chemical Activity

TL;DR

This paper demonstrates that complex trajectories of motile particles, including bacteria and artificial microswimmers, are universal and arise from their intrinsic nonlinear dynamics, independent of shape asymmetry or stochastic effects.

Contribution

It introduces a universal nonlinear framework explaining diverse motile trajectories without relying on shape asymmetry or randomness, supported by symmetry-based reduced equations.

Findings

Complex trajectories emerge naturally in simple fluid environments.

The model aligns with autophoretic particle dynamics.

Different motion patterns are explained by parameter variations.

Abstract

Locomotion is essential for living cells. It enables bacteria and algae to explore space for food, cancer to spread, and immune system to fight infections. Motile cells display trajectories of intriguing complexity, from regular (e.g. circular, helical, and so on) to irregular motions (run-tumble), the origin of which has remained elusive for over a century. This dynamics versatility is conventionally attributed to the shape asymmetry of the motile entity, to the suspending media, and/or to stochastic regulation. We propose here a universal approach highlighting that these movements are generic, occurring for a large class of cells and artificial microswimmers, without the need of invoking shape asymmetry nor stochasticity, but are encoded in their inherent nonlinear evolution. We show, in particular, that for a circular and spherical particle moving in a simple fluid, circular, helical and chaotic motions (akin to a persistent random walk) emerge naturally in different regions of parameter space. This establishes the operating principles for complex trajectories manifestation of motile systems, and offers a new vision with minimal ingredients. The reduced evolution equations based on symmetries are consistent with those derived for a model of an autophoretic particle including diffusion, emission/absorption at the particle surface and hydrodynamics, and provide qualitative and quantitative agreement.