A comparative study on the measurement of surface bubble size distributions in dry aqueous foams using optical methods

The measurement of bubble sizes in aqueous foams based on images of the surface is a typical method used in laboratory and industrial scales. In this article the relationship between the size distribution of the facet areas and the bubble size of wall-touching bubbles is investigated. To achieve this an invasive sampling approach is used for in-situ collection of wall-touching bubbles in dry foams, while the surface is imaged in parallel. Bubble and facet size distributions are obtained using automated image processing. It is shown that sharp peaks in the bubble size distribution will appear smoother in the facet size distribution. This results in an overestimation of polydispersity by the surface measurements. Furthermore, it is observed that the mean equivalent diameter of the facets is on average 6% smaller than the bubble diameter obtained using the sampling method. An approach proposed by Wang and Neethling (2009) gives a good approximation of the relation between facet and bubble size and can be used to reduce potential uncertainties in surface based bubble size measurements.