Speaker
Description
In biophysical systems, oriented locomotion is not imposed directly but emerges from the underlying chemistry. Molecules consume fuel, reactions proceed out of equilibrium, and somehow this chemical driving turns into persistent mechanical motion. The present work [1] addresses how to generate sustained mechanical activity from coupling to chemically driven stochastic dynamics, while maintaining a transparent structure of action, semi-reciprocal coupling, and local detailed balance. We study which characteristic features get transferred, and what new phenomena appear. To be specific, as illustrated in Fig. 1, we consider a heavy Newtonian probe (point particle) moving on a circle, coupled to a collection of fast independent but driven jump processes. Under time-scale separation, we treat the probe as the analogue of a Brownian particle bombarded by the nonequilibrium medium and derive the induced Langevin dynamics with explicit expressions for the streaming term, friction coefficient, and noise variance. These parameters are computed exactly in a weak coupling expansion. The induced friction is a sum of two terms: one entropic, proportional to the noise variance as in the Einstein relation for a thermal equilibrium bath, and a frenetic contribution that can take both signs. The frenetic part wins over a regime of parameters, making the total linear friction negative, and hence creating a linear instability. Detailed simulations confirm the initial growth driven by this anti-damping and exhibit a rich steady-state behavior (e.g., bimodal velocity distribution, U−shaped position distribution with peaks away from the minima of the potential), akin to active particles. Based on these numerical explorations, we conjecture which corrections to the induced Langevin equation are needed beyond time-scale separation to recover the numerical results.
Publications
[1] A. Beyen, F. Casini and C. Maes, A Rayleigh criterion for mechanical instability: inducing
activity by chemo–mechanical coupling (in preparation).