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The interfacial tension (IFT) is a critical parameter to inform our understanding of the phenomena of drop breakup and droplet–droplet coalescence in sheared water-in-diluted bitumen (dilbit) emulsions. A microfluidic extensional flow device (MEFD) was used to determine the IFT of the dilbit-water emulsion system for bitumen concentrations of 33%, 50%, and 67% by weight (solvent to bitumen ratio (S/B) = 2, 1, and 0.5, respectively) and two different pH values of water: 8.3 and 9.9. The IFT was observed to increase with the bitumen concentration and decrease significantly upon lowering the water pH. The time scale for achieving the steady state IFT increased with bitumen concentration and was less sensitive to the water pH. But the most important feature of our measurements is that the IFTs recorded were significantly smaller than the values reported in the literature. We recognized two important differences between our studies and prior investigations: measurement of the IFT of water drops in dilbit as opposed to dilbit drops in water in earlier studies, and time scales of measurement of IFT that ranged from hundreds of milliseconds to a few seconds, as compared to a minute or longer in past investigations. These differences were examined carefully, but neither was found to explain the low IFTs measured in our studies. Our work leads to the following hypothesis: the mechanical properties of the interface of a sheared water drop in bitumen are significantly different from a stagnant one.

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This study presents a semi-analytical solution for the problem of two touching drops with slipping interfaces pushed against each other in a uniaxial compressional flow at low capillary and Reynolds numbers. The jump in the tangential velocity at the liquid-liquid interface is modeled using the Navier slip condition. Analytical solutions of the contact force, the drop-scale stresses, and the drop-scale pressure are provided as functions of the slip coefficient α, the viscosity ratio κ, and the drop size ratio k. Since unequal drop sizes are considered, two problems are solved in the tangent sphere co-ordinate system to determine the steady state position: a pair of touching drops with its contact point at the origin of an axisymmetric straining flow, and two touching drops placed in a uniform flow parallel to the axis of symmetry of the drops. A general observation is that the effect of slip is manifested most strongly for drops whose viscosity is much greater than the suspending fluid κ≫1. For highly viscous drops, the flow and stress fields transition from those corresponding to solid particles for ακ ≪ 1, to those for inviscid drops in the limit ακ ≫ 1. The analytical expressions provided here for the contact force and the stress distributions will serve to provide the restrictions that complete the definition of the lubrication flow problem in the thin film between the two colliding drops. While the contact force that drains fluid out of the thin film is relatively unaffected by slip, the tangential stress and pressure in the near-contact region are mitigated significantly for ακ ≫ 1. The latter is expected to assist coalescence at high capillary numbers.