Sedimentation of an elliptical object in a two-dimensional foam

The sedimentation of an elliptical object in a dry two-dimensional, monodisperse foam is simulated. The calculations are quasi-static, allowing the identification and separation of the elastic, plastic and viscous response of the foam to the motion. In addition to its weight, the forces on the ellipse are due to the network of soap films and the pressures in the bubbles. These give rise to non-zero torque, lift and drag forces, causing the motion of the ellipse to deviate from a vertical path. Highly-stretched films are formed in the wake of the ellipse and asymmetry in the flow field, with bubbles moving from the front to the back of the ellipse along only one side, causes it to rotate from a metastable state with its major axis perpendicular to gravity into a stable orientation with its major axis parallel to the direction of gravity. When the orientation is intermediate between these two limits, there is a significant lift force which causes the ellipse to move laterally. A larger, more eccentric, ellipse rotates more quickly.