In search of correlations between the four-point measure of dynamic heterogeneity and other characteristics of glass-forming liquids under high pressure

In this paper, we examine the linear correlations earlier suggested between the four-point dynamic heterogeneity and the activation volume as well as between the four-point dynamic heterogeneity and the difference between isobaric and isochoric fragilities, which have been previously tested mainly at ambient pressure for different materials near the glass transition. We perform our analyses for twelve glass formers that belong to different material groups such as van der Waals liquids, polymer melts, protic ionic liquids, and strongly hydrogen-bonded liquids. Since our very recent finding [Phys. Rev. Lett. 111 (2013) 125701] shows that the dynamic heterogeneity decreases with increasing pressure along the glass transition line in the phase diagram, which is in contrast to the invariance of the shape of relaxation spectra in isochronal conditions, we exploit experimental data measured at ambient and elevated pressure to test whether the correlations occur over the whole phase diagram area where the supercooled liquid state is achieved. The obtained results are clarified theoretically in the limit of the estimate recently proposed for the degree of the dynamic heterogeneity by Berthier et al. [Science 310 (2005) 1797].