Decoding active force fluctuations from spatial trajectories of active systems

TL;DR

This paper presents a new method to accurately extract and analyze fluctuating active forces acting on particles within active systems, combining numerical validation and experimental application to bacterial baths.

Contribution

A novel technique for high-accuracy extraction of active force fluctuations from particle trajectories, applicable to both simulations and experimental data.

Findings

Successfully tested with numerical simulations of active systems.

Applied to experimental data of particles in bacterial baths.

Enables estimation of thermodynamic parameters in living matter.

Abstract

Mesoscopic active systems exhibit various unique behaviours - absent in passive systems - due to the forces generated by the corresponding constituents by converting their available free energies. However, estimating these forces - which are also stochastic and remain intertwined with the thermal noise - is especially non-trivial. Here, we introduce a technique to extract such fluctuating active forces acting on a passive particle immersed in an active bath with high statistical accuracy by filtering out the related thermal noise. We first test the efficacy of our method under numerical scenarios with different types of activity, and then apply it to the experimental trajectories of a microscopic particle (optically) trapped inside an active bath consisting of motile \textit{E.Coli.} bacteria. We believe that our simple yet powerful approach, which appears agnostic to the nature of the active force, should enable accurate measurement of force dynamics in living matter and potentially allow direct but reliable estimation of key thermodynamic parameters such as heat, work, and entropy production.