Particle orbits in a rotating liquid

The motion of small spherical particles under gravity, in a viscous fluid rotating uniformly about a horizontal axis, is investigated. Formulations and solutions are obtained for the particle orbit problem and the rotation rate optimization problem. It was found that the rotation rate which maximizes the fraction of the reactor cross-section area containing particles that will not spiral out to the wall in the experimental time (for heavy particles), or that have spiraled inward without hitting the wall (for light particles) is close to 1 rpm.