Investigation of the Force−Distance Relationship for a Small Liquid Drop Approaching a Liquid−Liquid Interface

Abstract

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.

Bubble vortex at surfaces of evaporating liquids

“…2. The apparatus is in common with an ordinary film balance for disjoining pressure measurements [15][16][17][18][19]. The film is formed by filling the ring with the liquid and subsequently depressurizing the meniscus.…”

Quantitative Analysis of Fluid Interface–Atomic Force Microscopy

“…In recent years, AFM experiments with deforming interfaces, like an air bubble (2-10) or oil droplet (11)(12)(13)(14)(15) submerged in water, have been met with limited success. Similar investigations with unique designs have made observations of a mercury drop flattening against mica (16) and of an oil drop impinging on a flat oil interface in water, i.e., the liquid surface forces apparatus (17). These have the added benefit over standard AFM of measuring the separation directly, as does Thompson's interferometric AFM for the air bubble.…”

Forces between two oil drops in aqueous solution measured by AFM

“…Some research has been undertaken on the interaction between liquid interfaces using other techniques, notably the liquid surface force apparatus (LSFA) [1][2][3] and a novel approach using magnetic particles dispersed inside an emulsion droplet [4][5][6][7][8]. These techniques have their strengths and weaknesses.…”