Abstract

In drainage experiments for liquid-gas foams, a sufficiently large liquid flow rate results in a downwards convection of bubbles. This 'wet', downwards-convecting region of foam can coexist with stationary 'dry' regions or with 'dry' regions that convect upwards. A possible explanation of this phenomenon is dilatancy. We introduce and develop a model that considers the dynamic dilatancy of a foam via force balances on a continuously sheared sample with a finite liquid fraction. Using microstructural information for the strain of typical foam structures (e.g. Kelvin and Weaire-Phelan foams) and the notion of stretching Plateau borders (i.e. foam channels) within a non-uniform bubble velocity field, the model can estimate the liquid content within a convective roll. Alternatively liquid content can be obtained via previously established relations between applied shear rate and foam osmotic pressure. The continuously sheared, downwards-convecting portion of foam is predicted to subsist at higher liquid content than an adjacent, unyielded, upwards-convecting portion of foam. Sustainable liquid content variations in the dynamic dilatancy model are comparable to or greater than those associated with static foam dilatancy. (C) 2008 Elsevier B.V. All rights reserved.