Understanding and accounting for different yielding rates in soft materials

Many soft materials show the transition from solid-like behavior to liquid-like behavior, but how this yielding transition occurs can vary significantly. Understanding the physics behind yielding is of great interest for the behavior of biological, environmental, and industrial materials. Some materials yield smoothly and gradually while others yield abruptly. We refer to this abrupt yielding as being “brittle”. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a sharp increase in loss modulus during oscillatory tests. We account for brittility within our recently proposed continuum model for yield stress materials (Phys. Rev. Lett. 126, (2021)). The original formulation describes the plastic viscosity as being dependent on the total strain rate; plastic flow is aided by the rate at which elastic deformation is acquired. We account for brittility by scaling the contribution of the recoverable component, which impacts the rate at which yielding occurs, by an amount related to the softness of the constituents relative to the bulk. The model predictions are successfully compared to results of different rheological protocols from several model yield stress fluids. Our study shows that the brittility, and therefore the softness of the constituents, plays a critical role in the transient nonlinear rheology of soft materials.