Characterisation of Surfactant and Protein Foams using a Polarised Light Scattering Technique

Abstract

Liquid foam is a two-phase dynamic system with a myriad of industrial applications. Recently a polarised light scattering technique has been developed as a possible method of continuous and non-invasive monitoring of the bulk foam properties. The overall objective of this research is to investigate the utilisation of a polarised light scattering technique on surfactant and protein foams, which are the two foams most commonly encountered in industries. Examples may include mineral flotation, food processing and pharmaceuticals. Based on the differences in the coarsening and drainage regimes, SDS (Sodium dodecyl sulphate, a low molecular weight ionic surfactant) and Casein (a protein) are selected as foaming agents to represent small and large surface active molecules. In the current work, a polarised light scattering experimental set-up has been designed to conduct experiments using foams with monodispersed and bidispersed bubble size distributions. Foams are initially generated under forced drainage to maintain a uniform axial liquid fraction profile. Subsequently, the foams enter a free drainage period, where the axial liquid fraction profile varies with time. During this free drainage period, two types of completely polarised light (vertical-linearly polarised light and left-handed circularly polarised light) are shone through the foam and the Stokes vector is measured. From this measurement, all the five polarisation characteristic parameters (degree of polarisation P, degree of linear polarisation PL, degree of circular polarisation PC, orientation angle ψ and ellipticity angle χ) of the scattered light are determined to examine the changes in the polarisation state due to the foam evolution. Six independent Mueller matrix elements (M11, M12, M22, M33, M34, M44), which contain the information of the optical and geometrical properties of foam, are determined via a combination of a series of optical components. At the same time, the liquid fraction and the bubble size distribution of the foam are independently measured by the pressure gradient method and the photographic method, respectively. The quantitative correlations between these Mueller matrix elements and the liquid fraction and the bubble size distribution of the foam are then investigated. During the free drainage of SDS foams, the bubble size distribution remains almost constant, although the liquid fraction decreases. This process can be divided into three stages based on the rate of the decrease in liquid fraction. This study has focused on the static stage where the rate of change in liquid fraction has reduced to nearly zero. Foams with three different bubble size distributions are investigated and it was found that four of the polarisation parameters, P, PL, PC and χ exhibit sensitivities to the liquid fraction and the bubble size distribution. ψ is largely independent of the liquid fraction and the bubble size distribution, but shows a certain degree of correlation with the structural rearrangement of the polydispersed foams. The liquid fraction affects all the six normalised independent Mueller matrix elements for all the three SDS foams. In particular, a simple quantitative correlation can be obtained between the liquid fraction and three normalised Mueller matrix elements N11, N22, N33, which reflect the differential scattering cross section, the bubble density and the capacity of retaining +45° polarised light of foams, respectively. In the case of Casein foams, it was found that they require higher proportions of liquid to maintain stability than SDS foams do. Along with the decrease in the liquid fraction, the bubble size distribution varies significantly due to coalescence. Therefore, multiple regression analysis was performed to investigate the individual effect of the liquid fraction and the bubble size distribution on the polarisation state. Similar to SDS foams, the variations of four of the polarisation parameters P, PL, PC and χ are shown to be associated with the liquid fraction and/or the bubble size distribution to different extents. The remaining polarisation parameter ψ, however, is completely independent of the liquid fraction and the bubble size distribution. A correlation between M11 & M22 from the Mueller matrix and the liquid fraction and the bubble size distribution with high correlation coefficient has been obtained. M12 shows dependence only on the bubble size distribution but no dependence on the liquid fraction. M33 is associated with both these foam parameters. For initially monodispersed Casein foams, M34 shows no dependence on the liquid fraction and the bubble size distribution, whereas M44 appears to be significantly correlated with the mean bubble diameter. For bidispersed Casein foam, both M34 and M44 are mainly associated with the liquid fraction. This work has shown that the polarised light scattering technique can be applied as a diagnostic tool to characterise surfactant and protein foams such as SDS and Casein foams and it can provide a basis for further foam studies using this technique.