Effects of fluid viscosity on band segregation dynamics in bidisperse granular slurries

Parallel experiments in long, axially rotated cylinders are used to study the influence of interstitial fluid viscosity on particle segregation in bidisperse granular slurries. A uniformly mixed initial state segregates into surface bands, which alternate between regions of large particles and regions composed of a mixture of small and large particles. As the tumbler rotates, the relative area of the mixed particle bands increases and saturates, while the number of bands reaches a peak and then decreases logarithmically in time for all viscosities studied. With increasing interstitial fluid viscosity, the asymptotic mixed band area increases proportionally, the time for bands to appear at the surface decreases, and the peak number of bands goes through a maximum at a viscosity of approximately 3 cP . Extrapolation to the low-viscosity limit matches the data for dry granular systems; at the high-end viscosity there is a value beyond which no axial banding occurs. A heuristic mechanism based on the coexistence of pure and mixed particle phases and their dependence on viscosity is presented to rationalize key aspects of the results.