Pattern Formation in a Rotating Aqueous Suspension

– A novel pattern forming instability in a mixture of a granular material and water in a horizontal rotating drum is experimentally investigated. The particles accumulate in radial symmetric rings separated by pure water. The transition between the homogeneous state and the structured state is hysteretic. The transition point is extrapolated from the growth rates of the chain of rings. The trajectory of a single particle is discussed to estimate an upper boundary for the transition point. Introduction. – Granular media show an interesting and peculiar dynamic behavior [1], and are also of major importance in technological processes. One of the standard problems of the powder technology is the mixing of materials [2]. It is often accompanied by pattern formation like stratification [3] or segregation [4, 5, 6, 7]. The axial and radial segregation in rotating-drum mixers with dry multicomponent mixtures, like small and large particles or particles of different densities, has been investigated. The different angles of repose of the two species are considered as the segregation mechanism for the axial segregation. The separation of large and small particles has recently been observed in a mixture of particles dispersed in a lower density fluid [8]. In the presence of a fluid the time for the pattern formation is significantly decreased. Granular media immersed in a fluid show a type of segregation even for monodisperse particles [9,10,11,12]. The volume fraction of granular particles in these experiments is much less than in the previous cases. Furthermore, the driving frequency of the drum is sufficiently high for the centrifugal forces to become dominant. The observed pattern is a chain of circumferential rings of high particle concentration. Tirumkudulu et al. reported on this type of axial segregation in partially filled horizontal cylinders [9, 11]. In both cases they used suspensions of particles, which were neutrally buoyant. If the device was completely filled with the suspension, no instability was observed at all. The experiment was modeled theoretically by Govindarajan et al. [13] including shear-induced diffusion of particles, concentrationdependent viscosity and the existence of a free surface. Boote and Thomas [10], and Thomas et al. [12] reported similar experiments with particle densities less and more dense than the surrounding fluid in a partially fluid-filled cylinder. For particles of a different density than the fluid, the free surface is not necessary for the instability. Lipson observed this phenomenon during crystal growth in a supersaturated solution [14].