Scale-dependent elasticity as a probe of universal heterogeneity in equilibrium amorphous solids

TL;DR

This paper investigates how the scale-dependent elastic shear modulus in amorphous solids reveals underlying heterogeneity, especially near the transition point, by linking it to the distribution of particle localization lengths using replica mean-field theory.

Contribution

It introduces a theoretical framework connecting mesoscale elasticity to the distribution of localization lengths in amorphous solids, highlighting scale-dependent softening near the transition.

Findings

Effective shear modulus softens at smaller scales.

Scale-dependent elasticity probes heterogeneity.

Response reveals localization-length distribution asymptotics.

Abstract

The equilibrium amorphous solid state -- formed, e.g., by adequately randomly crosslinking the constituents of a macromolecular fluid -- is a heterogeneous state characterized by a universal distribution of particle localization lengths. Near to the crosslink-density-controlled continuous amorphous-solidification transition, this distribution obeys a scaling form: it has a single peak at a lengthscale that diverges (along with the width of the distribution) as the transition is approached. The modulus controlling macroscale elastic shear deformations of the amorphous solid does not depend on the distribution of localization lengths. However, it is natural to anticipate that for deformations at progressively shorter lengthscales -- mesoscale deformations -- the effective modulus exhibits a scale-dependence, softening as the deformation lengthscale is reduced. This is because an increasing fraction of the localized particles are, in effect, liquid-like at the deformation lengthscale, and therefore less effective at contributing to the elastic response. In this paper, the relationship between the distribution of localization lengths and the scale-dependent elastic shear modulus is explored, and it is shown, within the setting of a replica mean-field theory, that the effective modulus does indeed exhibit scale-dependent softening. Through this softening, mesoscale elasticity provides a probe of the heterogeneity of the state as characterized by the distribution of localization lengths. In particular, the response to short-lengthscale elastic deformations is shown to shed light on the asymptotics of the universal localization-length distribution at short localization lengths.