Local plastic distortion of hard-sphere colloidal suspensions upon approaching the colloidal glass transition

Some complex systems, such as colloidal suspensions or foams, exhibit shear localization - regions of flow coexisting with regions of solid-like behavior when exposed to a local disturbance. The spatial extent of the flow region, functional form of the decay, and whether the transition from the flowing to the stagnant region is continuous or discontinuous depend on the particle interaction and packing fraction. We study the decay of azimuthal (tangential) velocity profiles with the distance from a local disturbance in hard-sphere colloidal suspensions upon approaching the colloidal glass transition. The local disturbance is from a dimer formed by two superparamagnetic particles, rotated by an external magnet. We study the dynamic response of the system by tracking individual colloidal particles using confocal microscopy. We find the tangential velocity decays approximately exponentially with the distance from the dimer center. However, the shear region is localized: beyond a certain distance, the colloidal sample appears to only strain elastically. As the colloidal particle density increases towards the colloidal glass transition, the length scale extracted from exponential fits to the velocity profiles also increases.