270409 Surface Chemistry and Van Der Waals Forces of the Air-Water and Oil-Water Interface: Homo- and Hetero- Interactions Between Drops and Bubbles
Understanding the interactions between droplets of oil and gas bubbles is central to tuning the desired properties of foodstuffs such as ice cream and mousse, cosmetics and pharmaceuticals, and in mineral flotation and separation. Fundamentally, the interactions between dissimilar fluid interfaces (drops and bubbles) or fluid interface with a solid are interesting problems. Dissimilar fluid interfaces may charge and deform to different extents. In addition, both dissimilar fluid interfaces and fluid interfaces interactions with solids can be intrinsically stable based on material choice not surface chemistry. We have used the atomic force microscope (AFM) to analyse collisions between bubbles and oil droplets - of around 100 microns diameter - in surfactant-free aqueous solutions  or a between a bubble and flat surfaces[2,3]. Homo- (bubble-bubble and drop-drop), hetero- (bubble- drop, bubble-solid) interactions were examined to show the role of colloidal and hydrodynamic forces that, together with interfacial deformations, dictate the stability in these systems.
Through carefully chosen experiments and quantitative modelling, it was possible determine important properties such as surface potentials and the Hamaker functions for the oil drops, air bubbles and surfaces. Between like pairs of drops or bubbles, electrical double-layer forces were predictably repulsive, and Van der Waals forces were attractive. However, it was found that within a small pH-range, double-layer forces between dissimilar bodies could be rendered attractive. Similarly, material choice allowed the sign of the Van der Waals force to be selected to be either positive or negative, and ionic strength also has a surprising role via screening when Van der Waals forces are small. Collisions at high speed show that local deformation of the bodies is controlled by their internal Laplace pressures, defining the profile of liquid film trapped between them.
These results give new insight into how coalescence and interactions can be controlled for fluid handling in microfluidics, as well as dispersion and formulation science. Material choice, pH and ionic strength can be used to enhance or avoid coalescence in hetero-bodied (dissimilar materials) systems, suggesting new, facile pathways for self-assembly at to add functionality to colloidal systems.
(1) Tabor, R. F.; Manica, R.; Chan, D. Y. C.; Grieser, F.; Dagastine, R. R., Repulsive van der waals forces in soft matter: Why bubbles do not stick to walls. Physical Review Letters 2011, 106, 064501.
(2) Tabor, R. F.; Morfa, A. J.; Grieser, F.; Chan, D. Y. C.; Dagastine, R. R., Effect of gold oxide in measurements of colloidal force. Langmuir 2011, 27, 6026-6030.
(3) Tabor, R. F.; Wu, C.; Lockie, H.; Manica, R.; Chan, D. Y. C.; Grieser, F.; Dagastine, R. R., Homo- and hetero-interactions between air bubbles and oil droplets measured by atomic force microscopy. Soft Matter 2011, 7, 8977-8983.