Modelling of foamed emulsion drainage

The drainage of foams created using emulsions has been investigated from both experimental and theoretical point of view. The drainage of emulsion foam is investigated using mixture of sodium dodecyl sulphate and oil, which were prepared using the double syringe method. For the preparation of each emulsion, SDS solution and oil are passed from one syringe into the other through a plastic tube leading to thorough mixing. In the course of drainage both the foam height and the thickness of the free liquid layer accumulated at the bottom of the foam were measured. A theoretical model was developed, taking into account both surface viscosity and non-Newtonian behaviour of the foamed emulsion to describe the time evolution of both the foam height and the thickness of the free liquid layer. The model is based on consideration of drainage of non-Newtonian liquid through the Plateau borders and the mobility of the gas/liquid interface is taken into account. Both experiments and theoretical predictions show no measurable change of the foam height while a free liquid layer starts to accumulate at the bottom boundary of the foam after an initial rapid increase of liquid volume fraction to the maximum value at the bottom of the foam. Theoretical predictions of rate of drainage, free liquid layer formation, foam height and liquid volume fraction for foamed emulsion systems of various oil volume fractions are compared with experimental observations. Comparison of the predicted and the experimentally measured time dependences showed a reasonable agreement.