(81c) The Dissolution of a Sheared Water Drop in Bitumen Measured Using a Novel Microfluidic Platform

It is well known that the dissolution rate and solubility of drops of one liquid emulsified in a second, immiscible liquid phase can be enhanced significantly by the presence of surfactants in the continuous phase. While methods for measuring solubility abound in the literature, measurement techniques for the dissolution rate are rare. In this study, we present a new, robust microfluidic platform for determining the dissolution dynamics of an individual drop in an immiscible suspending medium. We demonstrate this technique for emulsions of water in bitumen, which contains indigenous surfactants that enhance water solubilization. This is a difficult system for dissolution measurements due to the opacity of bitumen, but our shallow microfluidic channels allow us to clearly image the water-bitumen interface¹. Moreover, water-in-bitumen emulsions are extremely stable^1,2, and any sub-micron emulsified water droplets, which are difficult to remove even by ultracentrifugation, are also counted as solubilized water in traditional solubility measurements such as Karl Fischer titration. Our single drop dissolution technique avoids this complication.

Depending on the bitumen composition, water chemistry and flow conditions, the time for water dissolution in bitumen is governed either by interfacial phenomena or by mass transfer between the bulk bitumen phase and the water-bitumen interface. In the latter case, we can also calculate the solubility of water in bitumen, provided an independent measure of the size of the water-carrying species is available. Our study has uncovered four key features of the dissolution of water in bitumen under flowing conditions. First, increasing bitumen concentration enhances the water solubility, but the dissolution process slows down. Second, both dissolution rate and water solubility increase with increasing diluent aromaticity. Third, for a fixed strain rate of the flow past the drop, as the pH of the water drop increases, the dissolution rate of water diminishes. Fourth, water solubility decreases when asphaltenes are removed from the bitumen system. Our microscale results can predict the time required to completely solubilize a spherical drop in a suspending medium for a bench scale experiment, if the mixing conditions are specified. These and other distinguishing characteristics of our study will be elucidated in the presentation.

References

1) Sonthalia, R., Ng, S. and Ramachandran, A., 2016. Formation of extremely fine water droplets in sheared, concentrated bitumen solutions via surfactant-mediated tip streaming. Fuel, 180, pp.538-550.

2) Tchoukov, P., Yang, F., Xu, Z., Dabros, T., Czarnecki, J. and Sjöblom, J., 2014. Role of asphaltenes in stabilizing thin liquid emulsion films. Langmuir, 30(11), pp.3024-3033.