Probing the Fluctuation-Dissipation Theorem in a Perrin-like Experiment

TL;DR

This study introduces a novel experimental method to test the fluctuation-dissipation theorem in nonequilibrium colloidal systems, finding no violation and confirming the effective temperature matches the bath temperature.

Contribution

It presents a nonperturbative dual measurement technique to directly apply the FDT to colloidal tracers in aging gels, confirming theoretical predictions.

Findings

No violation of FDT observed across various conditions.

Effective temperature equals bath temperature in aging colloidal gels.

Method enables direct, single-sample FDT testing in complex fluids.

Abstract

In this Letter, we present a new experimental approach to investigate the effective temperature concept as a generalization of the fluctuation-dissipation theorem (FDT) for nonequilibrium systems. Simultaneous measurements of diffusion coefficient and sedimentation velocity of heavy colloids, embedded in a Laponite clay suspension, are performed with a fluorescence-recovery-based setup. This nonperturbative dual measurement, performed at a single time in a single sample, allows for a direct application of the FDT to the tracer velocity observable. It thus provides a well-defined derivation of the effective temperature in this ageing colloidal gel. For a wide range of concentrations and ageing times, we report no violation of the FDT, with effective temperature agreeing with bath temperature. This result is consistent with recent theoretical predictions on the coupling between the velocity observable and nonequilibrium gels dynamics.