This study examines the concentration and velocity patterns observed in a horizontal rotating cylinder completely filled with a monodisperse suspension of non-Brownian buoyant particles. The unique patterns or phases are mapped by varying both the rotation rate and the solvent viscosity. Individual phases are identified using both frontal ͑-z plane͒ and axial ͑r-plane͒ views. Phase boundaries are compared to those obtained recently for suspensions of nonbuoyant particles. Expressing the boundaries in terms of dimensionless parameters unifies the data for several samples at low rotation rates. When centrifugal force dominates, the behavior becomes quite different from previous studies.

We report a systematic experimental study of concentration and velocity patterns formed in a horizontal rotating cylinder filled completely with a monodisperse suspension of non-Brownian settling particles. The system shows a series of concentration and velocity patterns, or phases, with varying rotation rate and solvent viscosity. Individual phases are studied using both side and cross-sectional imaging to examine the detailed flow structures. The overall phase diagram of the system is mapped out as a function of the rotation rate and solvent viscosity. Attempts are made to analyze the functional form of the phase boundaries in order to understand the transition mechanism between different phases.