Effective temperatures from the fluctuation-dissipation measurements in soft glassy materials

TL;DR

This study tests the fluctuation-dissipation theorem in various non-equilibrium soft glassy materials using microrheology, finding no violations and questioning the applicability of the effective temperature concept.

Contribution

It provides comprehensive experimental validation of the fluctuation-dissipation theorem in soft glassy materials across a broad frequency range.

Findings

No violation of FDT in colloidal glasses and gels from 1 Hz to 10^4 Hz.

FDT holds even at low frequencies down to 0.1 Hz.

Questions the usefulness of effective temperature in these systems.

Abstract

We have investigated the validity of the fluctuation-dissipation theorem (FDT) and the applicability of the concept of effective temperature in a number of non-equilibrium soft glassy materials. Using a combination of passive and active microrheology to measure displacement fluctuations and the mechanical response function of probe particles embedded in the materials, we have directly tested the validity of the FDT. Our results show no violation of the FDT over several decades in frequency (1-10$^4$ Hz) for hard sphere colloidal glasses and colloidal glasses and gels of Laponite. We further extended the bandwidth of our measurements to lower frequencies (down to 0.1 Hz) using video microscopy to measure the displacement fluctuations, again without finding any deviations from the FDT.