Nature Materials (2019)
Understanding the complex entanglement between structural and dynamical properties of glasses is a great scientific challenge. A powerful technique to study such relations on a microscopic scale is microrheology, where one analyzes the translational dynamics of an externally driven probe particle. Here we experimentally investigate the mechanical properties of a colloidal glassy system by measuring simultaneously the translational and the rotational dynamics of an embedded active, i.e. self-propelled, probe particle (APP), whose orientational motion is unconstrained. We find that its rotational diffusion coefficient continuously increases towards the glass transition and drops down in the glassy state. Such unexpected behavior demonstrates a strong coupling mechanism between the APPs orientation and the glassy structure, which is in excellent agreement with a simple rheological model. Our results suggest the use of APPs as an efficient and versatile method to measure the micromechanical response of complex materials.