Effect of simple shear on liquid drainage within foams

This paper examines the impact of shear on the drainage of liquid through a foam. The effect on drainage of changes in the Plateau border network brought about by shear is examined. This effect of shear was modeled using both a detailed three-dimensional simulation of foam structure, and an idealized two-dimensional hexagonal foam. The main phenomenon that has been discovered is that shear induces an anisotropy in the drainage of liquid through the foam. If the foam is strained and liquid flows under gravity either perpendicular or parallel to the shear direction, a net liquid flow in the direction perpendicular to the direction of gravity is induced. It was found that the degree of anisotropy increases nearly linearly with increasing strain until the foam yields, after which the degree of flow anisotropy remains roughly constant. This is not a small effect, with the flow in the direction perpendicular to gravity being up to about 20% of the flow in the direction of gravity at the yield strain. This shear-induced anisotropy provides a potential explanation for the hitherto puzzling phenomenon of the convective roll in foam. The other two effects examined are the effect of shear on the length of Plateau borders per volume of foam, which increases as strain increases, and the resistance of the foam to flow in the direction of gravity, which increases if the strain direction is perpendicular to gravity, but decreases if the strain is parallel to gravity.