Rheology of draining steady-state foams

Aqueous foams continuously age due to fluid drainage and bubble coarsening, which makes it difficult to perform steady-state rheological measurements. Consequently we have developed the foam drainage rheology technique, where perfusion counteracts fluid drainage and bubble replenishment counteracts bubble coarsening during measurement of the shear stresses by a rheometer. We evaluate published power-law and Herschel-Bulkley models and find that parameters derived from emulsion experiments cannot describe our results. We propose a hybrid model, which combines our earlier film-shearing model, where the film thickness depends on liquid volume fraction, with a Herschel-Bulkley shear-rate dependence.