CM-AMO Seminar | Mind the Gap: a Cascade of Instabilities Created by Rotating Beads Near a Floor

Does a rotating bead always spin in place? Not if that bead is near a surface: rolling leads to translational motion, as well as very strong flows around the bead, even quite far away. These flows strongly couple the motion of nearby microrollers (rotating beads), which leads to a rich variety of collective effects. Using experiments in tandem with large-scale 3D simulations, we have shown that driving a compact group of microrollers leads to a new kind of flow instability, whose wavelength is controlled not by the driving torque or the fluid viscosity, but a geometric parameter: the microroller's distance above the container floor. Furthermore, under the right conditions, stable, compact clusters we term "critters" can emerge from the unstable interface. Our simulations and experiments suggest that these critters are a stable state of the system, move much faster than individual rollers, and quickly respond to a changing drive. We believe that critters are unique in that they are clusters which form only with hydrodynamic interactions; no interparticle potentials are needed to create these structures. Furthermore, as compact, self-assembled structures which can easily be remotely guided, critters may offer a promising tool for microscopic transport.