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

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. Following a discussion of intuitive expectations for the scale-dependent elasticity in the amorphous solid state, 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 behavior of the universal localization-length distribution at short localization lengths. Certain experimental techniques that have the potential to yield information specifically about the mesoscale structure and elasticity of amorphous solid states are discussed.