Investigation of the force–distance relationship for a small liquid drop approaching an adhesive liquid/liquid interface

Abstract

We have used a novel apparatus to investigate the forces between two liquid surfaces which are mutually attractive. An oil drop is formed at the tip of a thin flexible micropipette and the force exerted on the oil drop as it is pressed up to an oil/water interface is determined from the vertical deflection of the pipette 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 micropipette. The radius and the thickness of the film are derived from the optical interference pattern observed using a microscope. A theory is presented for the variation in force and film radius as the drop is moved up to and down from the interface. Experimental results are given for dodecane–water–dodecane emulsion films stabilised by the anionic surfactant sodium decyl sulfate (SDS) in the presence of different aqueous phase concentrations of NaCl, which modifies the attractive force between the monolayers. Data for the force and film radius as a function of the pipette position relative to the oil–water monolayer show qualitative agreement with theoretical predictions. It is shown that differences between theory and experiment are not due to the existence of a line tension at the perimeter of the thin emulsion film formed as the oil drop is pressed up to the oil/water interface. Discrepancies between theory and experiment may be due to imperfect ‘pinning’ of the three-phase contact line at the inner diameter of the micropipette.