Dynamic Correlation Length Scales under Isochronal Conditions

TL;DR

This study investigates how dynamic heterogeneity correlates with the slowing down of molecular motions in supercooled liquids, revealing that the number of dynamically correlated molecules depends mainly on relaxation time, regardless of temperature or pressure.

Contribution

First nonlinear dielectric experiments at high pressure measure dynamic correlation volumes, showing their primary dependence on relaxation time in non-associated liquids.

Findings

Dynamic correlation volume depends mainly on relaxation time.

Molecular dynamic simulations support minimal variation in density fluctuation susceptibility.

Heterogeneity likely drives the slowdown in supercooled liquids.

Abstract

The origin of the dramatic changes in the behavior of liquids as they approach their vitreous state - increases of many orders of magnitude in transport properties and dynamic time scales - is a major unsolved problem in condensed matter. These changes are accompanied by greater dynamic heterogeneity, which refers to both spatial variation and spatial correlation of molecular mobilities. The question is whether the changing dynamics is coupled to this heterogeneity; that is, does the latter cause the former? To address this we carried out the first nonlinear dielectric experiments at elevated hydrostatic pressures on two liquids, to measure the third-order harmonic component of their susceptibilities. We extract from this the number of dynamically correlated molecules for various state points, and find that the dynamic correlation volume for non-associated liquids depends primarily on the relaxation time, sensibly independent of temperature and pressure. We support this result by molecular dynamic simulations showing that the maximum in the four-point dynamic susceptibility of density fluctuations varies less than 10% for molecules that do not form hydrogen bonds. Our findings are consistent with dynamic heterogeneity serving as the principal control parameter for the slowing down of molecular motions in supercooled materials.