Foam stability: Part. II. Gas diffusion in foams

Abstract

AbstractPressure differences between foam bubbles of unequal size are the cause of gas transfer from small to larger bubbles. The rate at which the small bubbles decrease in size is calculated as a function of the radius of the bubbles, the surface tension of the liquid and the thickness and permeability of the liquid lamellae between the bubbles. The calculation is based on the assumption that the pressure difference between a small and an adjacent large bubble is dependent only, to a first approximation, on the radius of the smaller bubble.Values of the average lamella thickness calculated by means of the derived rate equation from the observed rate of shrinking of small bubbles in various foams, are of the right order of magnitude, if it is supposed that the permeability of the lamellae is determined by the solubility and the diffusion constant of the gas in the liquid.The rate equation enables the number of bubbles per unit volume of foam to be expressed as a function of time if the initial bubble size frequency distribution is known. It is shown that the bubble size distribution function can be calculated from the bubble sizes observed in the plane of contact between the foam and the glass wall of a container. An experimental method of determining the number and sizes of the bubbles in this plane is described.