Structures of Local Rearrangements in Soft Colloidal Glasses

We image local structural rearrangements in soft colloidal glasses under small periodic perturbations induced by thermal cycling. Local structural entropy ${S}_{2}$ positively correlates with observed rearrangements in colloidal glasses. The high ${S}_{2}$ values of the rearranging clusters in glasses indicate that fragile regions in glasses are structurally less correlated, similar to structural defects in crystalline solids. Slow-evolving high ${S}_{2}$ spots are capable of predicting local rearrangements long before the relaxations occur, while fluctuation-created high ${S}_{2}$ spots best correlate with local deformations right before the rearrangement events. Local free volumes are also found to correlate with particle rearrangements at extreme values, although the ability to identify relaxation sites is substantially lower than ${S}_{2}$. Our experiments provide an efficient structural identifier for the fragile regions in glasses and highlight the important role of structural correlations in the physics of glasses.