title: Hydrodynamic capture of microswimmers into sphere-bound orbits.
abstract: Self-propelled particles can exhibit surprising non-equilibrium behaviors, and how they interact with obstacles or boundaries remains an important open problem. Here we show that chemically propelled micro-rods can be captured, with little change in their speed, into close orbits around solid spheres resting on or near a horizontal plane. We show that this interaction between sphere and particle is short-range, occurring even for spheres smaller than the particle length, and for a variety of sphere materials. We consider a simple model, based on lubrication theory, of a force- and torque-free swimmer driven by a surface slip (the phoretic propulsion mechanism) and moving near a solid surface. The model demonstrates capture, or movement towards the surface, and yields speeds independent of distance. This study reveals the crucial aspects of activity–driven interactions of self-propelled particles with passive objects, and brings into question the use of colloidal tracers as probes of active matter.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Takagi
authors.forename: Jérémie
authors.initials: J
authors.lastname: Palacci
authors.forename: Adam B
authors.initials: AB
authors.lastname: Braunschweig
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Shelley
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
mesh_terms: D003102:Colloids / Q000737:chemistry; D057446:Hydrodynamics; D009068:Movement; D043942:Nanotubes / Q000737:chemistry*; D010316:Particle Size; D013499:Surface Properties; D014783:Viscosity
chemical_list: D003102:Colloids