Probing length scale growth near the colloidal glass transition using local plastic deformation*

Some complex materials—like colloidal suspensions and foams—demonstrate shear localization, where flowing regions coexist with areas exhibiting solid-like behavior in response to localized disturbances. The spatial reach of flow, the mathematical profile of velocity decay, and the nature of the transition from moving to stationary zones all depend on how particles interact and how densely they are packed. In this study, we analyze how tangential velocity diminishes with distance from a localized disturbance in hard-sphere colloidal suspensions approaching the colloidal glass transition. The disturbance is induced by a dimer composed of two superparamagnetic particles, which are rotated by an external magnetic field. The system's dynamic response is probed by tracking individual colloidal particles with confocal microscopy. Results show that tangential velocity decreases approximately exponentially with increasing distance from the dimer center. Notably, the shear zone remains localized: past a certain distance, the suspension responds with purely elastic strain. As the particle packing increases near the glass transition, the characteristic length scale obtained from exponential fits to the velocity profile grows correspondingly.