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Dynamic heterogeneity of glass-forming liquids in the density scaling regime

Recent analyses of high pressure measurement data suggest that the degree of the dynamic heterogeneity (χ_4)^max cannot be in general a single variable function of the structural relaxation time τ. For a wide class of real and model supercooled liquids, the molecular dynamics of which obeys a density scaling law at least to a good approximation, we argue that the important relation between the length and time scales that characterize molecular motions near the glass transition is controlled by a density factor. If a power law density scaling is valid for both the structural relaxation times and the degrees of the dynamic heterogeneity we find that the factor is a density power, the exponent of which is a measure of the observed decoupling between τ and (χ_4)^max. Then, the measure can be quantified by a difference between the power law density scaling exponents, which are usually different for τ and (χ_4)^max.