Sedimenting discs in a two-dimensional foam

The sedimentation of circular discs in a dry two-dimensional, monodisperse foam is studied. This, a variation of the classical Stokes experiment, provides a prototype experiment to study a foam's response. The interaction between two circular particles of equal size and weight is investigated as they fall through the foam under their own weight. Their positions are tracked and the lift and drag force measured in numerical calculations using the Surface Evolver. The initial placements of the discs are varied in each of two different initial configurations, one in which the discs are side by side and the second in which the discs are one above the other. It is shown that discs that are initially side-by-side rotate as a system during the descent in the foam. In the second scenario, the upper disc falls into the wake of the lower, after which the discs sediment as one with a constant non-zero separation. We present evidence that the foam screens this interaction for specific initial separations between the discs in both configurations. The force between a channel wall and a nearby sedimenting disc is also investigated. (C) 2008 Elsevier B.V. All rights reserved.