(601h) Microfluidic Measurement of the Interfacial Tension of the Water-Diluted Bitumen Interface

After the froth treatment section in the processing of oil sands, the product – diluted bitumen or dilbit, still contains about 2 to 5 % by weight of water. The difficulty in removing the residual water originates from the extremely fine sizes of the water droplets (less than 10 microns). The interfacial tension is a critical parameter to inform our understanding of drop breakup and droplet-droplet coalescence, which together govern the drop size distribution in sheared water-in-dilbit emulsions. A microfluidic extensional flow device was used to determine the interfacial tension of the dilbit - water emulsion system for bitumen concentrations of 33%, 50% and 67% (solvent to bitumen ratio (S/B) = 2, 1 and 0.5, respectively) by weight, and two different pH values of water: 8.3 and 9.9. The interfacial tension 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. A key observation in our measurements is that the interfacial tensions reported here are significantly smaller than the values reported in the literature. After eliminating the obvious doubt of flow-induced segregation of surfactants leading to larger deformations and predicting lower interfacial tensions, we recognized two other important differences from prior studies. First, our measurement of the interfacial tension was done for the system of water drops in dilbit, as opposed to the system of dilbit drops in water employed in earlier studies. Second, the time scales of measurement of the interfacial tension 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 interfacial tensions measured in our studies. Our work leads ultimately to the following hypothesis: the mechanical properties of a sheared water-bitumen interface can be significantly different from a stagnant one. Possible physical mechanisms of this hypothesis will be discussed.