(9g) Shape-Directed Assembly of Active Colloidal Crystals

While the role of particle shape is well-understood in the equilibrium self-assembly of colloidal crystals, less is known about the effects of shape when active colloids assemble out of equilibrium. In this work, we seek to understand how particle shape directs the structure of assemblies of rigid colloidal swimmers. We fabricate bimetallic gold-platinum microplates with arbitrary cross-sections using projection photolithography and physical vapor deposition. These plates swim in hydrogen peroxide and assemble into 2D colloidal crystals within several seconds. This rapid assembly arises from the activity of the plates and attractive hydrodynamic interactions between plates. Through experiments, we systematically investigate the assembly behavior of plates with regular polygonal cross-sections and circular disks. By changing the cross-sectional shape of the plates, we control the crystal structure and quality. Our results suggest that particle shape is a powerful handle to control the self-assembly of active colloids. Looking forward, we see shape as an avenue to program the dynamics of colloidal machines.