THE YIELDING TRANSITION IN SOFT AMORPHOUS SOLIDS UNDER OSCILLATORY SHEAR: FROM MICROSCOPIC REARRANGEMENTS TO MACROSCOPIC FAILURE

Complex fluids such as polymer solutions, colloidal suspensions, emulsions, or foams are ubiquitous in our daily lives. These soft materials often display a non-trivial response to mechanical perturbations, which depends both on the time scale and the amplitude of the applied perturbation. In particular, yield stress fluids are a wide class of soft materials that respond like solids to small stresses but exhibit fluid-like behavior when the applied stress exceeds a certain threshold value (the yield stress). Beyond its major relevance in various applications from the cosmetic to the food industry, the yielding transition of these soft materials is considered to be a convenient experimental model for the more general phenomenon of mechanical failure in amorphous solids. By exploiting a custom shear-cell coupled to a standard bright-field microscope, we provide a unique multiscale analysis of the yielding transition under oscillatory shear overcoming the limitation of conventional rheological measurements. Through a novel acquisition protocol and image analysis scheme, we are able to accurately monitor shear-induced structural rearrangements within the material and characterize the mesoscopic deformation field, while simultaneously measuring the rheological response. The investigation of three distinct yield stress materials revealed a complex interplay between brittleness, shear band formation, and features of the shear-induced microscopic dynamics. Specifically, for the most ductile material, we observe Gaussian and non-cooperative particle dynamics, contrasting with the non-Gaussian and cooperative dynamics, and the occurrence of shear bands observed for the most brittle one. These findings offer valuable insights into material behavior under dynamic conditions, paving the way for a deeper understanding of yielding phenomena.