Micron-scale tensiometry for studying density-matched and highly viscous fluids — with application to bitumen-in-water emulsions

Measuring the interfacial tension (IFT) between density-matched fluids has been a serious challenge in the study of tensiometry. These measurements can be further complicated when one or both of the fluids possess high viscosities. In this study, a micron-scale technique is developed to circumvent such difficulties. This microscopic technique involves stretching an otherwise spherical drop, of diameter ∼10 μm in an aqueous medium, with the use of two suction micropipettes; one of the pipettes is shaped as a cantilever to allow for measurement of the stretching force. It is shown that, for mechanical experiments conducted on the 1–10 μm scale, as in the present application, the gravitational body force and viscous effects can be neglected (provided relaxation times of several seconds are allowed). The Young–Laplace equation, which describes the force-drop deformation relation, is utilized to determine the equilibrium IFT. The system of present interest was that of bitumen drops in water at room temperature, in which the densities of the two phases are nearly equal (to within 1%) and the viscosity of bitumen is extremely high (more than 105 times that of water). This is the first study of bitumen–water (IFT) at room temperature over a range of pH.