A novel approach for estimating the average bubble size for foams flowing in vertical columns

Bubbles in flowing foams deform, coalesce, and rupture. The bubble size, however, determines to a great extent the functionality and mechanical properties of the foam. Often it is not possible to measure directly the bubble size in a flowing foam, either because the system or the foam itself is not transparent. In order to determine the average bubble size in a flowing foam, a novel approach to estimate the average bubble size was developed, based upon the overflowing foam liquid content and conductivity measurements. A flowing foam column with variable orifice diameters was designed and experiments were performed with different gas flowrates, orifice diameters and surfactant (Sodium dodecyl sulphate) concentrations. Image analysis was used to measure directly the bubble size in the foam. When progressively smaller orifices were inserted, more coalescence was observed, as well as a decrease in the overflowing liquid content and the foam conductivity. Estimating the bubble size from these measurements showed close agreement with the directly measured bubble sizes.