H igh grade synthetic crude oil is produced from the bitumen found in northern Alberta's oil sands deposits. Before raw bitumen could be upgraded into a feed product for the refineries, it must first be 'extracted' from the sand ore (that is, bitumen must be separated from the sand grains, clays particles and connate water). At present, the most viable method of extracting bitumen is by a flotation process in which mined oil sands, after being slurried with warm or hot water, is fed into a flotation vessel. Once in the vessel, the unwanted solids will settle and be eliminated through the underflow stream, while bitumen -which enters the vessel as small aerated droplets -is collected at the top as a froth. The success of this flotation process depends critically on the tendency of the bitumen droplets to coalesce in aqueous environments. (Coalescence gives rise to bigger droplets, and in turn enhances significantly their rising velocities according to Stokes law; in addition, the quality of a froth is determined by the propensity of the oil drops to coalesce into an oil-continuous dispersion (Shaw et al., 1996).) As the flotation process is one which is continuous in time, an understanding of its fundamental mechanisms must involve studying the dynamics of bitumen-bitumen coalescence. The rate of coalescence between two bitumen droplets in water, it is clear, is driven by the bitumen-water interfacial tension σ bw and limited by the viscosity of oil phase µ b . (The aqueous phase is much less viscous than bitumen and can therefore be treated as inviscid.) Measurements of these two fundamental properties, unfortunately, have not been straightforward: The determination of σ bw is hindered by a vanishingly small density difference between the two immiscible phases (see Moran et al. (2000) for further discussion). This difficulty, however, is recently overcome by the use of a microcantilever technique which operates at the low Bond number limit (Moran et al., 1999;Moran et al., 2000). What remains -and which is the focus of this study -is to determine the bulk viscosity of bitumen. Although a seemingly simple task which has long been carried out, the measurement of µ b is in fact still plagued by problems which arise from effects of dissipative heating -especially at low temperatures when the viscosity of bitumen can be as large as ~10 6 times that of water. (In the following, low temperature will imply temperatures between 20 and 30°C.) With the ever-increasing need in the oil sands industry to operate at minimal energy consumption (for economic as well as environmental reasons), there is a renewed incentive to accurately determine the viscosity of bitumen at temperatures down to around 20°C. Here, using a drop shape recovery technique which avoids dissipative heating altogether, we show that previous measurements of µ b at low tempera-
* Author to whom correspondence may be addressed. E-mail addre s s : tony.yeung@ualberta.caIn determining the viscosity of bitumen using conventional methods, the measurements must be...