Stress Overshoot in Startup Flow of Soft Particle Glasses in Experiments and Large-Scale Computer Simulations

Soft particle glasses (SPG), which are jammed beyond the random close-packing fraction of equivalent hard spheres, show rich rheology under shear flow. Here we study the startup flow of SPGs using three-dimensional particle-dynamics large-scale simulations and experiments at different particle packing fractions and shear rates. Experimentally, two distinct behaviors can occur depending on the shear rate. At low shear rates, the stress increases monotonically up to steady flow, revealing ductile behavior. When the shear rate increases, the shear stress initially undergoes an overshoot before decreasing and plateauing at a steady-state value. The strain at which SPGs glass yields is close to 0.1 and increases with the shear rate. The magnitude of the stress overshoot increases with the aging of the SPGs. Large scale computer simulations are used to rationalize these experimental observations. They also give evidence for the microstructural changes associated with the overshoot in terms of particle compression and orientation along the compression direction of the flow. Using different materials, we show that both the experimental and simulation peak stress and peak strain are described by universal variations as a function of a dimensionless shear rate. These results provide new routes to tailor the transient behavior of concentrated suspensions in the complex flow situations involved in many processes.