Inertial Self-Caging: Dynamics of Macroscopic Swimmers at Moderate Reynolds Number Sustaining Chemical Wake Resonance

TL;DR

This study explores the complex dynamics of macroscopic chemically driven swimmers at moderate Reynolds numbers, revealing stable resonant states, chemical self-feedback effects, and a transition to stochastic behavior.

Contribution

It demonstrates, through experiments and modeling, the existence of stable resonant states and the impact of chemical self-feedback in inertial regimes, advancing understanding of high-speed active matter.

Findings

Identification of stable resonant states under confinement

Observation of exponential speed decay with oscillations

Discovery of a threshold speed leading to stochastic behavior

Abstract

Self-propelled phoretic swimmers are generally studied in the laminar flow regime, where their low speed renders inertial effects negligible and trajectories highly predictable. This research tackles the challenge of propulsion in the inertial regime, at moderate Reynolds numbers (100 < Re < 200), where fluid dynamics becomes non-linear. By using a chemically driven macroscopic hydrogel this work demonstrates, through experiments and modeling, the existence of stable resonant states under confined geometry: as the swimmer circles, it interacts with its own lasting chemical wake. This chemical self-feedback creates a complex, stable motion characterized by both universal exponential speed decay with superimposed significant periodic speed oscillations. Furthermore, a critical threshold speed is identified, where the system abruptly transitions from the resonant oscillatory regime to a stochastic stop & go behavior. These findings provide a fundamental understanding of how chemical fields, hydrodynamic inertia, and confinement couple to determine the motion properties of high-speed active matter.