| Amorphous materials show intriguing flow and arrest phenomena that are of central importance in engineering and industrial applications. Recent years have yielded new promising scaling concepts to describe the flow of amorphous solids, and new soft glassy model systems to test these concepts on large length and time scales. In particular, colloidal and granular model systems have advanced our understanding of internal relaxation mechanisms of strained glasses by direct imaging of the internal particle-scale dynamics. I will focus on the role of microscopic correlations in the relaxation: By combining rheological measurement with microscopic imaging or scattering, we follow correlations of the internal strain field non-affine displacements. When the material relaxes under small applied stress, the flow is neither macroscopically uniform nor strongly localized: We observe system-spanning strain correlations that reveal a novel mechanical criticality at the transition from rigidity to flow. These correlations are isotropic when thermal rearrangements dominate at small applied stress, but become anisotropic when the applied stress increases. These new, anisotropic correlations cause the flow to localize, ultimately leading to material failure. While our model systems allow direct imaging of these critical strain fluctuations, we expect similar mechanisms to hold for other soft glassy materials as well as atomic and molecular glasses. |