Proposed lower bound for the shear viscosity to entropy density ratio in some dense liquids

TL;DR

This paper evaluates Kovtun et al.'s proposed lower bound for the shear viscosity to entropy density ratio in dense liquids by analyzing experimental data, confirming that real liquids exceed this theoretical limit.

Contribution

The study tests the proposed bound using experimental data on various dense liquids, providing empirical support for its validity.

Findings

All examined dense liquids exceed the proposed bound.

The shear viscosity of alkali metals scales onto a universal curve.

Experimental data supports the theoretical lower bound.

Abstract

Starting from relativistic quantum field theories, Kovtun et al. (2005) have quite recently proposed a lower bound eta/s >= hbar /(4 pi kB), where eta is the shear viscosity and s the volume density of entropy for dense liquids. If their proposal can eventually be proved, then this would provide key theoretical underpinning to earlier semiempirical proposals on the relation between a transport coefficient eta and a thermodynamic quantity s. Here, we examine largely experimental data on some dense liquids, the insulators nitrogen, water, and ammonia, plus the alkali metals, where the shear viscosity eta(T) for the four heaviest alkalis is known to scale onto an `almost universal' curve, following the work of Tankeshwar and March a decade ago. So far, all known results for both insulating and metallic dense liquids correctly exceed the lower bound prediction of Kovtun et al.