Tiny stir bar reveals growing length scale as the colloidal glass transition is approached

Colloidal suspensions serve as a model system for studying the glass transition. Using confocal microscopy, we track the motion of colloidal particles to examine the system's microscopic behavior as it approaches the glass transition. In our experiments, we introduce a small number of paramagnetic beads into a dense suspension of hard-sphere colloidal particles. These beads occasionally form pairs, or dimers, which we can manipulate using an external permanent magnet to induce rotation. The rotating dimer acts as a localized perturbation within the colloidal suspension, allowing us to investigate how the effects of this disturbance change when the concentration of the colloidal suspension and the rotational speed of the dimer are varied. We aim to gain deeper insights into the nature of the glass transition by probing the system's response to these localized disturbances. We find that the tangential velocity decays nearly exponentially with the distance from the center of the local disturbance. The decay is discontinuous, indicating a transition between the sheared region near the dimer and the jammed state at some critical distance away from the dimer. As the volume fraction increases towards the colloidal glass transition, the length scale extracted from exponential fits to the velocity profiles also increases.