Surface Forces in Thin Liquid Films

Thin liquid films (TLFs) of water are ubiquitous in daily lives as well as in many industrial processes. They can be formed between two identical phases, as in colloid films between two macroscopic surfaces and foam films between two air bubbles; and between two dissimilar phases, as in wetting films. Stability of the colloids, foams, and wetting films is determined by the surface forces in the TLFs. Depending on the nature of the surfaces involved, the stabilities can be predicted using combinations of three different forces, i.e., the van der Waals, electrical double layer (EDL), and hydrophobic forces. The objective of the present work is to study the roles of these forces in determining the stabilities of the TLFs of water confined between i) an air bubble and a hydrophobic surface and ii) an oil drop and a hydrophobic surface, with particular interest in studying the role of the hydrophobic force.

The first part of the study involves the measurement of the surface forces in the TLFs confined between bitumen drops and mineral surfaces. Deformation of bitumen drops has been monitored by interferometry while it approaches a flat surface. By analyzing the spatiotemporal film profiles, both the capillary and hydrodynamic forces have been calculated using the Young-Laplace equation and the Reynolds lubrication approximation, respectively, with the surface forces being determined by subtracting the latter from the former. The results are useful for better understanding the effects of electrolyte and pH on bitumen liberation and recovery by flotation and for developing a filtration model from first principles.

The second part of the study involves the surface force measurement in wetting (flotation) films. Surface forces in the TLFs of water on silica surfaces have been measured using the force apparatus for deformable surfaces (FADS) using an air bubble as a force sensor. The measurements have been conducted in the presence of various cationic surfactants such as dodecylamine hydrochloride (DAH), and alkyltrimethylammonium chloride (CnTACl), electrolytes, and polymers. The results show that film stability and hence the kinetics of film thinning can be greatly improved by the control of bubble ζ-potentials, whose role in flotation has long been neglected in flotation studies.

Force measurements have also been conducted in the TLFs of water confined between oil drops and hydrophobic surfaces. Stability of this type of film plays an important role in a process of using oil drops rather than air bubbles to collect hydrophobic particles from aqueous phase. The force measurements conducted in the present work show that hydrophobic forces are much stronger in water films formed between oil drops and hydrophobic surfaces than in water films formed between air bubbles and hydrophobic surfaces, which can be attributed to the differences in the Hamaker constants involved.