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 ca n be further complicated when one or both of the fluids possess high viscos ities. In this study, a micron-scale technique is developed to circumvent s uch difficulties. This microscopic technique involves stretching an otherwi se spherical drop, of diameter similar to 10 mu m in an aqueous medium, wit h 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 th at, for mechanical experiments conducted on the 1-10 mu m scale, as in the present application, the gravitational body force and viscous effects can b e neglected (provided relaxation times of several seconds are allowed). The Young-Laplace equation, which describes the force-drop deformation relatio n, is utilized to determine the equilibrium IFT. The system of present inte rest was that of bitumen drops in water at room temperature, in which the d ensities of the two phases are nearly equal (to within 1%) and the viscosit y of bitumen is extremely high (more than 10(5) times that of water). This is the first study of bitumen-water (IFT) at room temperature over a range of pH. (C) 2000 Elsevier Science B.V. All rights reserved.