The motion of a rough particle in a Stokes flow adjacent to a boundary

Results are presented of experimental investigations into the motion of a heavy sphere in a rotating cylinder which is completely filled with highly viscous fluid. For a given cylinder rotation rate, the sphere adopts a fixed position and rotates adjacent to the cylinder wall. For the case of a smooth sphere the motion is consistent with that predicted by a Stokes flow model. Artificially roughened spheres exhibit particle–boundary contact caused by impacts of surface asperities with the boundary for low cylinder surface speeds. For higher cylinder surface speeds the behaviour of the roughened spheres crosses smoothly from the particle–boundary contact regime to motion with hydrodynamically lubricated flow.