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 betw een two liquid surfaces. In the configuration described here, an oil d rop is formed at the tip of a thin flexible micropipet. The force exer ted on the oil drop as it is pressed up to an oil-water interface is d etermined from the deflection of the pipet shaft. The disjoining press ure in the thin, oil-water-oil emulsion film formed by contact of the drop with the oil-water interface is determined by the hydrostatic pre ssure applied to the oil contained in the micropipet. The radius and t he thickness of the film is derived from the optical interference patt ern 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 in terface. Experimental results are given for dodecane-water-dodecane em ulsion films stabilized by the anionic surfactant sodium bis(2-ethylhe xyl) sulfosuccinate (AOT). Data for the force and film radius as a fun ction of the pipet position relative to the oil-water monolayer show g ood agreement with theoretical predictions. The variation of disjoinin g pressure with film thickness for the emulsion films is in accord wit h electrostatic theory.