Free surface instability in a confined suspension jet

A suspension confined between two close parallel plates is studied in the Stokesian regime. The use of boundary integral equations and the lubrication approximation allows to compute the hydrodynamic forces acting on the particles. The forces are long ranged and depend on the orientation of the relative position and velocity of particles. This tensorial character predicts an ‘‘antidrag’’ that is observed in experiments. The effect of the computed hydrodynamic forces is studied in the dynamics of a jet of particles falling by a gravitational field, which shows a surface instability similar to the Kelvin–Helmholtz one. A theoretical model, based on hydrodynamic-like equations, is able to predict the instability that is produced by the interaction of the long-range forces and the free surface. r 2005 Elsevier B.V. All rights reserved. Suspensions, that is, solid particles immersed in a fluid—such as sand in water or dust in air—have been extensively studied in physics and engineering science. Their applications are wide, and a few examples are fluidized beds, pneumatic flow or sedimentation. Recent experiments show a wide variety of phenomena, for example the break-up of a blob of particles [1–3], the pattern formation in a rotating suspension [4], and a suspension jet [3]. All of these show the importance of a microscopic description of suspensions. Confined suspensions between two parallel see front matter r 2005 Elsevier B.V. All rights reserved. .physa.2005.05.036 nding author. dress: [email protected] (R. Soto).