Local shear transformations in hard-sphere colloidal glasses

Although defect-mediated plastic deformation in crystals has been well-understood for decades, understanding equivalent processes in glasses remains an area of active research. Theory and simulation predict the existence of "shear defects" or "shear transformation zones" (STZs) that mediate plastic deformation in glasses as dislocations do in crystals, but direct observations of these defects in amorphous structures remain elusive. An ideal experiment would catch the defects in action by observing the 3D, real-time motion of every microscopic constituent in a macroscopic glass sample. Colloidal glasses provides a unique experimental system in which we can realize such an ideal experiment and directly study structures, defects, and dynamics of amorphous materials across the complete range of relevant length and time scales. We analyze particle-level trajectories and local strain fields obtained from confocal microscopy experiments on ~1-μm-diameter, hard-sphere colloidal glasses under conditions of quiescence or uniform shear deformation. We find that shear transformation zones are active in both sheared and quiescent colloidal glasses, and we examine the evolution of the STZ population with time as well as increasing macroscopic strain. On strain reversal, we observe partial elastic recovery, followed by plastic deformation that compensates for irreversibly transformed regions. We further identify individual STZs directly by comparing their measured local strain fields to ideal Eshelby inclusions, fit for their positions and sizes, and investigate how their free volume, local density, coordination, and other structural predictors evolve leading up to and after the shear transformation.