Memory-less response and violation of the fluctuation-dissipation theorem in colloids suspended in an active bath

Claudio Maggi,Roberto Di Leonardo,Matteo Paoluzzi,Luca Angelani

doi:10.1038/s41598-017-17900-2

Claudio Maggi, Roberto Di Leonardo + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-17900-2

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Abstract

We investigate experimentally and numerically the stochastic dynamics and the time-dependent response of colloids subject to a small external perturbation in a dense bath of motile E. coli bacteria. The external field is a magnetic field acting on a superparamagnetic microbead suspended in an active medium. The measured linear response reveals an instantaneous friction kernel despite the complexity of the bacterial bath. By comparing the mean squared displacement and the response function we detect a clear violation of the fluctuation dissipation theorem.

Highlights

The dynamics and statistical mechanics of self-propelled particles is attracting a considerable attention both from the fundamental point of view and for its potential applications[1,2,3,4]
The random propulsion force relaxes on finite time-scale which depends on the specific propulsion mechanism, for example the propulsion velocity in swimming wild-type E. coli changes abruptly during “tumbles”[7], while in chemically propelled Janus particles the swimming direction changes gradually because of rotational diffusion[8]
We have studied numerically and experimentally the time-dependent response and fluctuations of a particle immersed in a bacterial bath

Summary

Introduction

The dynamics and statistical mechanics of self-propelled particles is attracting a considerable attention both from the fundamental point of view and for its potential applications[1,2,3,4]. The random propulsion force relaxes on finite time-scale which depends on the specific propulsion mechanism, for example the propulsion velocity in swimming wild-type E. coli changes abruptly during “tumbles”[7], while in chemically propelled Janus particles the swimming direction changes gradually because of rotational diffusion[8]. This combination of an instantaneous response and “colored” noise leads, by construction, to a non-equilibrium dynamics which violates the second Kubo fluctuation dissipation theorem[9] (FDT) already at the level of individual active particles.

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