Phys. Rev. Lett. 113, 208301 (2014)

Shear banding — the localization of deformations in bands — is common to metallic, polymer and colloidal glasses. But how such shear inhomogeneities are related to the glasses' microscopic structure and dynamics remains unclear. In atomic and molecular glasses, structural and dynamical changes at the atomic scale are difficult to observe, even by state-of-the-art electron microscopy; however, in colloidal systems the long time trajectories of individual colloids can be easily tracked. Indeed, by using confocal microscopy, Peter Schall and colleagues have been able to investigate the microscopic underpinnings of shear banding in a model colloidal glass. In previous work, they had shown that the non-monotonic flow curves associated with shear inhomogeneities relates to the local structure of the colloidal glass. Now, the researchers found that above a certain critical shear rate, the glass separates into two coexisting steady states with different diffusion timescales, and that such phase separation is analogous to the first-order transition between two phases in equilibrium. It would not be surprising if such a dynamic transition occurs in metallic glasses as well.