Forces between a rigid probe particle and a liquid interface: comparison between experiment and theory

Forces between a silica colloidal particle probe and an immobilized decane droplet in a range of surfactant (sodium dodecyl sulfate,SDS)solution concentrations have been measured using the atomic force microscope. Independent measurements of the droplet ú potential, colloidal probe surface potential, contact angle, interfacial tension, and probe radius are used as inputs into the interpretive theory developed by Chan et al. (J. Colloid Interface Sci. 2001, 236, 141) to provide a quantitative understanding of the measured forces operating at liquid-liquid interfaces. The theory is used to determine the true separation between the solid and the deformable liquid interface. It provides the link between interfacial deformation and disjoining pressure due to electrical double layer and van der Waals interactions in contributing to the observed force in these experiments. Within experimental error, the theory is able to account for the force vs displacement data obtained from atomic force microscopy measurements. It is shown that, in the presence of SDS, the interaction is always repulsive. As a result of the deformation of the droplet shape, forces up to 5 times larger than those observed between solid surfaces are seen. Variations in the interfacial tension of the deformable droplet have the largest effect on the measured forces