The effect of surfactant on foam and froth properties

The presence of a surfactant in a gas-liquid system causes a substantial change in dispersion properties. Creation of fresh surface, where the concentration of surfactant is lower, leads to a local increase in surface tension. These surface tension changes can be conveniently investigated in an Overflowing Cylinder apparatus, at similar rates of surface expansion, O(1) s(-1), to those encountered during film drainage in foam. A model of the diffusion and adsorption of surfactant leads to an expression for the dependence of surface tension change on the rate of surface expansion, and on the surfactant properties. Larger values of this surface tension change are associated with more stable foams. As reported by van Voorst Vader et al(1)., there is an intermediate concentration at which foamability is most severe. As the gas flow rate is increased, the basic flow regime changes from uniform bubbling to froth to spray. The presence of a surfactant can cause the uniform bubbling regime to change to cellular foam. In this regime the bubble size depends largely on the orifice Reynolds number, and a natural foam fractionation occurs, causing higher surfactant concentrations at the top of the foam. At higher flow rates froth is encountered, with both large, fast bubbles, and a population of smaller bubbles whose size decreases as the flow rate increases, due to increased shear. Predictions of bubble size do not take enough account of the unaerated liquid height, which affects circulation, or the foaming properties of the liquid.