Particle motion in a liquid film rimming the inside of a partially filled rotating cylinder

Abstract

Both heavier- and lighter-than-liquid particles will float on liquid-air surfaces. Capillary forces cause the particles to cluster in typical situations identified here. This kind of clustering causes particles to segregate into islands and bands of high concentrations in thin liquid films rimming the inside of a slowly rotating cylinder partially filled with liquid. A second regime of particle segregation, driven by secondary motions induced by off-center gas bubbles in a more rapidly rotating cylinder at higher filling levels, is identified. A third regime of segregation of bidisperse suspensions is found in which two layers of heavier-than-liquid particles that stratify when there is no rotation, segregate into alternate bands of particles when there is rotation * . I. Capillary forces The deformation of the air-liquid interface due to floating light particles or due to trapped heavy small particles gives rise to capillary forces on the particles. These forces may be qualitatively understood from simple arguments. Two kinds of forces act on particles: forces due to gravity and forces due to the action of contact angles. These two kinds of forces are at play in the vertical force balance but require a somewhat more elaborate explanation for horizontal force balance. The effects of gravity are usually paramount for heavier-than-liquid floating particles in which one particle will fall into the depression of the second. A heavier-than-liquid particle will fall down a downward sloping meniscus while an upward buoyant particle will rise. Capillary forces cause particles to cluster as shown in figure 1.