Violation of the fluctuation-dissipation relation during the formation of a colloidal glass

Abstract

The relationship between the conductivity and the polarization noise is measured in a gel as a function of frequency in the range 1Hz − 40Hz. It is found that at the beginning of the transition from a fluid like sol to a solid like gel the fluctuation dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. At the lowest frequencies of the measuring range it persists for times which are about 5% of the time needed to form the gel. This phenomenology is quite close to the recent theoretical predictions done for the violation of the fluctuation dissipation theorem in glassy systems.Many physical systems in nature are not in thermodynamic equilibrium because they present very slow relaxation processes. A typical example of this phenomenon is the aging of glassy materials: when they are quenched from above their glass transition temperature T g to a temperature T < T g , any response function of these systems depends on the aging time t a spent at T . For example, the dielectric and elastic constants of polymers continue to evolve several years after the quench [1]. Because of these slow relaxation processes, the glass is out of equilibrium, and usual thermodynamics does not apply. However, as this time evolution is slow, some concepts of the classical approach may be useful for understanding the glass aging properties. A widely studied question, is how the temperature of these systems can be defined. One possible answer comes from the study of the deviation to the Fluctuation Dissipation Theorem (FDT) in an out of equilibrium system (for a review see ref. [2,3,4]). In this letter we show that this approach is relevant for the study of a sol-gel transition, where a strong violation of FDT is measured. Implications of this observation go beyond the physics interest. Indeed FDT is used as a tool to extract, from 1

Measurement of Effective Temperatures in an Aging Colloidal Glass

“…We measure the equal-time fluctuations of this local oscillator and show that, in an ergodic phase, the temperature of the oscillator T o equals the environment temperature T bath of the system, as required by equilibrium statistical mechanics. Significantly, when we repeat the measurements in an aging glass we find a higher temperature and T o > T bath .All experiments to date on colloidal glasses [3,4,5] have relied on active, driven measurements. We conduct our experiments instead in a quasi-static limit which has several advantages.…”

“…Experiments have so far produced conflicting results. Studies of colloidal glasses have reported that T eff increases [3], remains unchanged [4], or even decreases [5] with age in contrast to measurements on structural [6] and spin glasses [7] which have revealed effective temperature warmer than the bath temperature.In this Letter, we report the temperature of a micrometer-sized sphere immersed in an aging colloidal suspension, as the suspension transforms from a fluid to a glass. The particle, captured in an optical trap, constitutes a microscopic harmonic oscillator whose fluctuations probe the nonequilibrium dynamics of the aging glass.…”

“…Experiments have so far produced conflicting results. Studies of colloidal glasses have reported that T eff increases [3], remains unchanged [4], or even decreases [5] with age in contrast to measurements on structural [6] and spin glasses [7] which have revealed effective temperature warmer than the bath temperature.…”

“…All experiments to date on colloidal glasses [3,4,5] have relied on active, driven measurements. We conduct our experiments instead in a quasi-static limit which has several advantages.…”

Dynamic particle tracking reveals the ageing temperature of a colloidal glass

“…Although theoretical results have flourished, the difficulties in the experimental testing of the fundamental predictions of the theories have hampered the development of an understanding of ageing in glasses [14][15][16][17][18] . Experiments so far have shown conflicting results, which are usually masked by large intermittent fluctuations in the observables 17 ; a behaviour which seems beyond the current theoretical formalisms 2 .…”