Accounting for brittle yielding in soft materials

Many soft materials yield, changing their mechanical response from that of a soft solid to that of a viscous fluid, but how this transition occurs can vary significantly. Understanding the physics behind this transition is of great interest for the behavior of biological, environmental, and industrial materials. Some materials yield gradually, while others yield more abruptly and are referred to as being "brittle". The key rheo-physical signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a sharp increase in the loss modulus during oscillatory tests. We account for brittility with a small modification to our recently proposed continuum model for yield stress materials (Kamani et al., Phys. Rev. Lett. 126, (2021)). We account for brittility by modifying the contribution of the recoverable component to the total strain rate, which impacts the rate at which yielding occurs. The model results are successfully compared to results of different rheological protocols from a number of model yield stress fluids having different microstructures, indicating the generality of the approach. Our study shows that the brittility of soft materials can be described as a continuum, and plays a critical role in determining the rate of yielding transition.