On the forced drainage of foam

The method of so-called ‘forced drainage’ has been extensively used to characterise the steady one-dimensional drainage of gas–liquid foams. In this paper, it is shown that the liquid drainage rate is approximately independent of surface dilational viscosity and surface tension (insofar as it creates capillary suction). However, liquid drainage rate is dependent on surface shear viscosity and Marangoni stresses. For transient foam drainage, an independence upon surface dilational viscosity is not demonstrated. Previously reported forced drainage data have been described in a dimensionally consistent form so that different studies can be directly compared with one another, and this has revealed two curious anomalies in the data, some of which may be due to previously reported, but generally overlooked, multi-dimensional effects in forced drainage experiments.