Overshoots in stress strain curves: Colloid experiments and schematic mode coupling theory

TL;DR

This paper investigates stress overshoot phenomena in dense colloidal dispersions under shear flow through experiments, simulations, and mode coupling theory, revealing their microscopic origin and providing a unified schematic model.

Contribution

The authors develop and validate a schematic mode coupling theory model that captures stress overshoot phenomena and links microscopic correlations to macroscopic rheological behavior.

Findings

Stress overshoot occurs in dense colloids during shear.

The schematic model accurately describes stress-strain behavior.

Transient stress correlations exhibit a plateau and negative decay.

Abstract

The stress versus strain curves in dense colloidal dispersions under start-up shear flow are investigated combining experiments on model core-shell microgels, computer simulations of hard disk mixtures, and mode coupling theory. In dense fluid and glassy states, the transient stresses exhibit first a linear increase with the accumulated strain, then a maximum ('stress overshoot') for strain values around 5%, before finally approaching the stationary value, which makes up the flow curve. These phenomena arise in well-equilibrated systems and for homogeneous flows, indicating that they are generic phenomena of the shear-driven transient structural relaxation. Microscopic mode coupling theory (generalized to flowing states by integration through the transients) derives them from the transient stress correlations, which first exhibit a plateau (corresponding to the solid-like elastic shear modulus) at intermediate times, and then negative stress correlations during the final decay. We introduce and validate a schematic model within mode coupling theory which captures all of these phenomena and handily can be used to jointly analyse linear and large-amplitude moduli, flow curves, and stress-strain curves. This is done by introducing a new strain- and time-dependent vertex into the relation between the the generalized shear modulus and the transient density correlator.