  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.
