Liquid is More Rigid than Solid in a High-Frequency Region

TL;DR

This paper demonstrates that liquids can be more rigid than solids at high frequencies due to universal response properties, challenging traditional views on material rigidity.

Contribution

It reveals that high-frequency rigidity of liquids surpasses that of solids, based on universal response functions applicable across various material classes.

Findings

Liquids become more rigid than solids at high frequencies.

Rigidity comparison depends on extrinsic factors like pinning center density.

The result is supported by nonequilibrium molecular dynamics simulations.

Abstract

We compare rigidity of materials in two phases, liquid and solid phases. As a measure of the rigidity, we employ the one characterizing how firmly the material is fixed by low density of pinning centers, such as impurities and rough surfaces of walls, against a weak force. Although a solid is more rigid than a liquid against a low-frequency force, we find that against a high-frequency force the liquid becomes more rigid than the solid of the same material. Since this result is derived from universal properties of a response function, it is valid for wide classes of materials, including quantum and classical systems and crystalline and amorphous solids. An instructive example is studied using nonequilibrium molecular dynamics simulations. We find that the frequency region in which a solid is more flexible than a liquid is not purely determined by intrinsic properties of the solid. It depends also on extrinsic factors such as the density of pinning centers.