On the density scaling of liquid dynamics

TL;DR

This paper investigates the superpositioning of relaxation data in liquids, demonstrating that the scaling behavior aligns with an inverse power law potential when using reduced quantities, especially relevant at higher temperatures.

Contribution

It clarifies the importance of using reduced quantities for scaling analysis and explains discrepancies at higher temperatures in liquid dynamics.

Findings

Scaling holds for approximately 100 liquids and polymers.

Differences between reduced and unreduced scaling are negligible in supercooled regime.

Using reduced quantities relates scaling exponent to intermolecular potential.

Abstract

Superpositioning of relaxation data as a function of the product variable TV^{\gamma}, where T is temperature, V the specific volume, and {\gamma} a material constant, is an experimental fact demonstrated for approximately 100 liquids and polymers. Such scaling behavior would result from the intermolecular potential having the form of an inverse power law (IPL), suggesting that an IPL is a good approximation for certain relaxation properties over the relevant range of intermolecular distances. However, the derivation of the scaling property of an IPL liquid is based on reduced quantities, for example, the reduced relaxation time equal to T^1/2V^-1/3 times the actual relaxation time. The difference between scaling using reduced rather than unreduced units is negligible in the supercooled regime; however, at higher temperature the difference can be substantial, accounting for the purported breakdown of the scaling and giving rise to different values of the scaling exponent. Only the {\gamma} obtained using reduced quantities can be sensibly related to the intermolecular potential.