Recent advances in the atomic force microscopy (AFM) force measurement techniques have allowed the direct measurement and theoretical interpretation of the interaction between a liquid droplet and a solid surface or two liquid droplets.
1 Here we investigate the interaction across an aqueous thin film between fluorocarbon oil droplets, hydrocarbon droplets or a droplet and a flat solid in the absence of stabilizers. The initial interest of the study was to measure the short ranged van der Waals interaction forces operating in these systems. The van der Waals interaction is usually attractive, but it can become repulsive when two dissimilar phases interact through a third phase of intermediate optical properties.
2 Preliminary results between fluorocarbon droplets and solid surfaces did show repulsion, but forces between symmetric oil droplets also show repulsion, even at high electrolyte concentrations. These repulsive interactions also exhibited a pH dependence. Theoretical analysis of the magnitude and the range of these forces suggest that DLVO forces can not explain the observed behaviour. The force behaviour will be discussed in the context of ion adsorption and charging effects at bare oil water interfaces.
1. Dagastine, R.R, Manica, R., Carnie, S.L., Chan, D.Y.C., Stevens, G.W., Grieser, F. Science, 2006, 313, 210.
2. Milling, A., Mulvaney, P., and Larson, I., J. Colloid and Interface Sci., 1996, 180, 460.