Bubble entrainment by a sphere falling through a horizontal soap foam

Abstract

Processes such as particle separation, froth flotation and explosion suppression rely on the extent to which particles are trapped by foam films. We simulate the quasi-static motion of a spherical particle through a stable, horizontal soap film. The soap film subtends a fixed contact angle, in the range , where it meets the particle. The tension and pressure forces acting on the particle are calculated in two cases: when the film is held within a vertical cylinder, trapping a bubble but otherwise free to move vertically, and when the outer rim of the film is held in a fixed circular wire frame. Film deformation is greater in the second case, and the duration of the interaction therefore increases, increasing the contact time between particle and film. As the soap film returns towards its equilibrium shape following the passage of the particle a small bubble is trapped for contact angles below a threshold value of 90°. We quantify how the size of this bubble increases when the particle is larger and when the contact angle is smaller.