Michelle Driscoll1
Northwestern University1
Does a rotating bead always spin in place? Not if that bead is small and near a surface: in this case, rotating leads to translational motion, as well as fast flows around the bead, even quite far away. These flows cause strong hydrodynamic coupling to nearby microrollers (rotating beads), which leads to a rich variety of collective effects. This system is experimentally realized by using magnetically actuated colloidal particles, and we find that these collective effects manifest as a wide array of emergent structure from shocks, to fingering patterns, to stable, hydrodynamically-bound clusters. We study these emergent structures using a variety of computational and experimental techniques, including Stokesian dynamics simulations and a coupled magnetic-optical imaging system. We find that in all cases, the scale of the emergent structures is set by a single geometric parameter: the height of the microroller above the surface. Recently, we have begun to study the interaction of microrollers with a structured environment, for example a forest of fixed obstacles or a sea of passive particles. In the case of fixed obstacles, we find a strong hydrodynamic trapping effect occurs in our experimental system, and we have used our simulation methods to characterize this trapping in detail. When the obstacles are free to move, we observe that the microrollers restructure the landscape around them, at length scales many times the size the roller.