Theory of suspension segregation in partially filled horizontal rotating cylinders

It is shown that a suspension of particles in a partially-filled, horizonta l, rotating cylinder is linearly unstable towards axial segregation and an undulation of the free surface at large enough particle concentrations. Rel ying on the shear-induced diffusion of particles, concentration-dependent v iscosity, and the existence of a free surface, our theory provides an expla nation of the experiments of Tirumkudulu [Phys. Fluids 11, 507 (1999); 12, 1615 (2000)]. (C) 2001 American Institute of Physics.