Plastic or Viscous? A Reappraisal of Yielding in Soft Matter

TL;DR

This paper critically examines the concept of yield stress in soft jammed materials, clarifying the differences between rheological and solid mechanics perspectives to resolve terminological ambiguities.

Contribution

It provides a comprehensive analysis contrasting rheological and solid mechanics views on plasticity and yield stress in soft matter.

Findings

Highlights the distinction between flow and irreversible deformation.

Clarifies the ambiguous use of 'yield stress' in soft matter rheology.

Proposes a unified framework for understanding yielding in soft materials.

Abstract

Many soft jammed materials, such as pastes, gels, concentrated emulsions, and suspensions, possess a threshold stress, known as yield stress, that must be exceeded to cause permanent deformation or flow. In rheology, the term plastic flow is commonly used to describe continuous flow (unbounded increase in strain with time) that a material undergoes above a yield stress threshold. However, in solid mechanics, plasticity refers to irreversible but finite, rate-independent deformation (strain that does not evolve with time). In addition, many soft materials exhibit viscosity bifurcation, a prominent thixotropic signature, which further complicates the definition and interpretation of yield stress. The threshold stress at which viscosity bifurcation occurs is also termed a yield stress, even though deformation below this threshold is not purely elastic, while above this threshold, the material flows homogeneously with a constant shear rate. This paper revisits these critical issues by analyzing the rheological and solid mechanics perspectives on plasticity. The insights presented here are intended to address certain terminological ambiguities for interpreting flow in soft jammed materials.