Quantifying single oil-particle interactions in aqueous media

Abstract

Detailed knowledge about the single-pair interactions between a rigid micro-particle and a liquid drop or air bubble immersed in some fluid is one way to predict and/or design the ensemble behavior of mixed colloidal materials, emulsions and other hybrid dispersions, from a kind of first-principles approach. The invention of the atomic force microscope (AFM) has made it possible to accomplish the direct measurement of forces from the interaction of a colloidal particle with a solid object, bubble or droplet. However, there are several fundamental difficulties with a compliant sample that must be addressed before colloidal microscopy can be applied quantitatively to academic studies and practical applications. The purpose of this work is to develop a general method for approaching AFM investigations of highly deformable fluid interfaces and to demonstrate its usefulness in specific areas of interest.These studies are the first to quantify film drainage and rupture times of aqueous films with AFM between a rigid micro-particle (toner) and an oil drop under dynamic conditions similar to paper recycling. It is also shown that electrosteric stabilization of the oil/water interface due to adsorbing cationic polyelectrolytes may be reversed via lipophilic surfactant addition to the oil. Other investigations of more idealized systems show the necessity of detailed AFM experiments to determine unknown interaction parameters for hydrophobically-driven phenomena. The so-called electrolyte titration is developed to control the net attraction between a hydrophobized-silica sphere and plate, revealing a characteristic curve of probe snap-in distance with concentration that allows accurate fitting of multiple surface forces parameters. From this knowledge, a theoretical force analysis of a sphere deforming a curved fluid interface is developed and tested on an oil drop with a polystyrene sphere in water. Fluid interface (FI)-AFM is then adapted to reveal the complexities of the hydrodynamics in a spherically wrapping thin film and of the effects of changing interfacial tension with ionic surfactants.