Space-resolved stress correlation and viscoelastic moduli for polydisperse system: two faces of the stress noise

Abstract

We discuss several advances in the theory of space-resolved stress correlations and viscoelastic relaxation moduli for liquids and other amorphous systems. Our study focuses on three aspects: (i) For a given wavevector q the q-dependent longitudinal (parallel to q) and transverse (perpendicular to q) viscoelastic moduli of a fluid relax to equilibrium values in the long time limit t ! 1. We derive relations for the q-dependent equilibrium moduli in terms of generalized structure factors, which are valid for systems with mass polydispersity. (ii) We revisit an earlier derivation of the relation between the q-dependent tensors of viscoelastic relaxation moduli (E) and stress correlations (C), which employed the concept of the “stress noise”, but was also hinged on consideration of non-stationary flows. The new derivation of the C-E relation presented here is based on a conceptually simple argument avoiding non-stationary processes. (iii) We discuss the relevance of the mass current field for the q-dependent viscoelastic relaxation moduli and the general relations between these moduli and correlation functions of the stress noise.