Emergence of bimodal motility in active droplets

TL;DR

This paper demonstrates how autophoretic droplets can spontaneously switch between different motility modes due to nonlinear interactions of chemical and hydrodynamic fields, revealing insights into complex biological-like locomotion strategies.

Contribution

It introduces a minimal isotropic droplet model that exhibits spontaneous bimodal gait switching driven by viscosity-dependent nonlinear coupling.

Findings

Bimodal gait switching occurs spontaneously in autophoretic droplets.

Higher hydrodynamic modes are excited with increased viscosity.

Droplet interaction with chemical gradients leads to anomalous diffusive motion.

Abstract

To explore and react to their environment, living micro-swimmers have developed sophisticated strategies for locomotion - in particular, motility with multiple gaits. To understand the physical principles associated with such a behavioural variability,synthetic model systems capable of mimicking it are needed. Here, we demonstrate bimodal gait switching in autophoretic droplet swimmers. This minimal experimental system is isotropic at rest, a symmetry that can be spontaneously broken due to the nonlinear coupling between hydrodynamic and chemical fields, inducing a variety of flow patterns that lead to different propulsive modes. We report a dynamical transition from quasi-ballistic to bimodal chaotic motion, controlled by the viscosity of the swimming medium. By simultaneous visualisation of the chemical and hydrodynamic fields, supported quantitatively by an advection-diffusion model, we show that higher hydrodynamic modes become excitable with increasing viscosity, while the recurrent mode-switching is driven by the droplet's interaction with self-generated chemical gradients. We further demonstrate that this gradient interaction results in anomalous diffusive swimming akin to self-avoiding spatial exploration strategies observed in nature.