Microscopic Mechanisms of Plastic Deformation in Colloidal Glass

Although defect-mediated plasticity in crystals has been well understood for decades, understanding equivalent processes in glasses remains an area of active research. In particular, global plastic deformation in crystals can be accounted for by adding up the local contribution of each individual dislocation to the bulk strain; this quantitative connection between local and global deformation is known as the "Orowan Relation." Theory and simulation predict, and experiments support, the existence of "shear defects" or "shear transformation zones" (STZs) in glasses that play an analogous role to dislocations in crystals, but direct observations of individual STZs and measurements of their contribution to the bulk strain remain challenging. Colloidal glasses provide a unique experimental system in which we can directly study structures, defects, and dynamics of amorphous materials. In this work, we analyze particle-level trajectories and local strain fields obtained from confocal microscopy experiments on 1.55-μm-diameter, hard-sphere colloidal glasses under conditions of uniform shear deformation. We develop quantitative methods to identify individual shear defects in the glass during deformation and characterize their location, size, and individual contribution to the global strain. We find that these STZs account for the entire applied strain, thus demonstrating an Orowan Relation for glass.