Spatial correlations in the dynamics of glassforming liquids: Experimental determination of their temperature dependence

TL;DR

This study experimentally measures how the number of dynamically correlated molecules in supercooled liquids changes with temperature, revealing growth near the glass transition and a power-law regime at higher temperatures.

Contribution

It introduces a method to experimentally determine the temperature dependence of dynamic correlations in glass-forming liquids using three-point susceptibilities.

Findings

N_corr increases as temperature approaches the glass transition.

N_corr exhibits a power-law dependence on relaxation time at higher temperatures.

Dynamic response to density is smaller but similar to the response to temperature.

Abstract

We use recently introduced three-point dynamic susceptibilities to obtain an experimental determination of the temperature evolution of the number of molecules, N_corr, that are dynamically correlated during the structural relaxation of supercooled liquids. We first discuss in detail the physical content of three-point functions that relate the sensitivity of the averaged two-time dynamics to external control parameters (such as temperature or density), as well as their connection to the more standard four-point dynamic susceptibility associated with dynamical heterogeneities. We then demonstrate that these functions can be experimentally determined with a good precision. We gather available data to obtain the temperature dependence of N_corr for a large number of supercooled liquids over a wide range of relaxation timescales from the glass transition up to the onset of slow dynamics. We find that N_corr systematically grows when approaching the glass transition. It does so in a modest manner close to the glass transition, which is consistent with an activation-based picture of the dynamics in glassforming materials. For higher temperatures, there appears to be a regime where N_corr behaves as a power-law of the relaxation time. Finally, we find that the dynamic response to density, while being smaller than the dynamic response to temperature, behaves similarly, in agreement with theoretical expectations.