Recent experiments in a partially filled horizontal Couette device [Tirumkudulu et al., Phys. Fluids 11, 507 (1999)] have revealed a new instability in the flow of suspensions containing narrow-sized neutrally buoyant particles in a very viscous Newtonian liquid where the suspension under shear segregates into regions of high and low particle concentration along the length of the cylinder. We conducted similar experiments but in a different geometry, viz., a horizontal cylinder, in which the cylinder was partially filled with that same suspension and rotated about its horizontal axis. We found that, within a certain range of parameter space, the initially uniform suspension divided into cylindrical bands of high particle concentration separated by regions of pure liquid. Although it seems likely that this instability is initiated by particle concentration fluctuations, which, in turn, lead to fluctuations in the effective viscosity of the suspension, we have been unable to provide a quantitative explanation for this phenomenon.
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Tirumkudulu et al. (2000) studied this question.
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