Investigation of the force-distance relationship for a small liquid drop approaching a liquid-liquid interface

We describe a novel apparatus for the investigation of the forces between two liquid surfaces. In the configuration described here, an oil drop is formed at the tip of a thin flexible micropipet. The force exerted on the oil drop as it is pressed up to an oil-water interface is determined from the deflection of the pipet shaft. The disjoining pressure in the thin, oil-water-oil emulsion film formed by contact of the drop with the oil-water interface is determined by the hydrostatic pressure applied to the oil contained in the micropipet. The radius and the thickness of the film is derived from the optical interference pattern observed using a microscope. A simple theory is presented for the variation in force and film radius as the drop is moved up into the interface. Experimental results are given for dodecane-water-dodecane emulsion films stabilized by the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT). Data for the force and film radius as a function of the pipet position relative to the oil-water monolayer show good agreement with theoretical predictions. The variation of disjoining pressure with film thickness for the emulsion films is in accord with electrostatic theory.