Drainage and stability of foam films during bubble coalescence in aqueous salt solutions

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Bubble coalescence and thin liquid films (TLFs) between bubbles known as foam films, are central to many daily activities, both natural and industrial. They govern a number of important processes such as foam fractionation, oil recovery from tar sands and mineral recovery by flotation using air bubbles. TLFs are known to be stabilized by some salts and bubble coalescence in saline water can be inhibited at salt concentrations above a critical (transition) concentration. However, the mechanisms of the inhibiting effect of these salts are as yet contentious. The aim of this work is to characterize the behavior of saline liquid films both experimentally and theoretically to better understand the mechanisms. The effect of sodium halide and alkali metal salts including NaF, NaBr, NaI, NaCl, KCl and LiCl on the stability of a foam film was investigated by applying the TLF interferometry method. To mimic realistic conditions of bubble coalescence in separation processes, the drainage and stability of TLFs were studied under non-zero bubble (air-liquid interface) approach speeds (10-300 µm/s) utilizing a nano-pump. For each of the salts studied, a critical concentration (Ccr) above which liquid films lasted for up to 50 s depending on the salt type, concentration and the interface approach speed, was determined. For concentrations below Ccr, the saline liquid films either ruptured instantly or lasted for less than 0.2 s. Ccr follows the order NaF<LiCl<NaCl<KCl<NaBr<NaI and was found to be independent of the bubble approach speed in the investigated range of speeds. For TLFs of deionised water, a critical speed of 35 µm/s, above which water films ruptured rapidly, was obtained. To develop insight into mechanisms of the inhibiting effect of salts, a theoretical investigation was also undertaken. Three models available in the literature to predict Ccr were critically examined. The first two models employed the non-retarded and retarded van der Waals attractions to evaluate the Ccr from the discriminant of quadratic or cubic polynomials. The third model modified the previous models following the same approach, and replaced van der Waals attractions with hydration repulsions of water molecules. Their model predictions depend on the rupture thickness of liquid films which is usually unknown and requires further experimental work to be determined. To resolve the uncertainty in the first two models concerning the non-retarded and retarded Hamaker constants, the Lifshitz theory on the van der Waals interaction energy and the available spectrum for water dielectric permittivity was applied to determine the Hamaker constants for saline liquid films. Values of A = 3.979 X 10-20J and B = 3.492 X 10-29J.m were obtained for non-retarded and retarded Hamaker constants respectively. The value for the retarded Hamaker constant is almost one order of magnitude larger than what was considered in the second model. Employing the new values for Hamaker constants in the models, it was shown that the role of van der Waals attractions in bubble coalescence in salt solutions is much less significant than previously hypothesized. Therefore another attractive disjoining pressure which is stronger than the van der Waals attraction is required to predict the experimental Ccr. To modify the models a novel methodology was proposed which resolves the mathematical uncertainties in modelling the Ccr and can explicitly predict it from any relevant intermolecular forces. The generality of the novel methodology was validated by re-applying the theory to establish the previous models obtained by the discriminant method. It was shown further that the third mode is physically inconsistent and not only the repulsive hydration disjoining pressure, but any other repulsive disjoining pressure, cannot be the main driving force for inhibiting bubble coalescence in salt solutions. Using the novel methodology the model was modified by incorporating hydrophobic attractions. The model suggested by Eriksson et al. (1989) for hydrophobic attractions was employed in the modelling to predict the experimental Ccr. The hydrophobic strength and decay length in Eriksson et al.’s model were determined by using the experimental results for the Ccr and thickness. A power law correlation was proposed for the hydrophobic constant of different salt types as a function of the salting-out coefficient as representative of salt type. The predicted Ccr values are in a good agreement with the experimental data obtained in this thesis and in the literature. It is envisaged that the outcomes of this work will contribute to a better understanding of the mechanisms responsible for the stability of foam films between two bubbles in saline water. The proposed novel methodology allows further improvements to the models for predicting Ccr values, for example, by incorporating the effect of inertia. The proposed correlation for the hydrophobic constant could also be modified based on the force measurement data for foam films of salt solutions.