Dynamics and rheology of 2D colloidal crystals with active anisotropic impurities

Active motion at complex fluid-fluid interfaces is a ubiquitous phenomenon in nature. However, an intriguing question that is not fully addressed is how active motion affects and gets influenced by its complex environment. Here, we design a 2D colloidal crystal containing active particles as a model system to understand the mechanics of complex interfaces in the presence of activity. We characterize the dynamics, rheology and phase behavior of 2D colloidal crystals formed using spherical polystyrene (PS) particles in the presence of active PS-Platinum Janus particles. In the presence of activity, the overall crystal becomes dynamic with a heterogeneous spatial distribution of disorder. Through particle-tracking microrheology and interfacial shear rheology, we report the discovery of a phase transition from solid-like to liquid-like interface driven by the active motion of a small number of active impurities in the crystal. The local perturbations induced by the active impurities have long-range effects on the dynamics of particles in 2D colloidal crystals modifying their overall viscoelasticity. The correlations between microstructure and dynamics from our experiments can provide insights into the behavior of a broad range of complex systems such as, motion of particles at oil-water interfaces in Pickering emulsion microreactors and the motion of molecular motors on cell membranes.