Evolution of Properties and Microstructure in Water-in-Oil Emulsions Subjected to Shear

Crude oil slicks subjected to shearing by surface waves form water-in-oil emulsions. The viscosity these emulsions decreases with increasing shear strain rate, suggesting that the shearing alters their microstructure. To elucidate the mechanisms involved, we simultaneously measure the evolution of the droplets size and spatial distributions as well as the viscoelastic properties of several emulsions made from light to heavy oils. The experiments are performed in a specially designed, transparent, Taylor–Couette rheometer that allows in situ microstructural observation. It has a 40 cm diameter, fixed, transparent outer cylinder, and a rotating inner cylinder, which is mounted on precision bearing, and equipped with a torquemeter. The narrow (2mm) gap between cylinders and the range of speeds involved assure that the flow remains laminar at strain rates ranging between 0.01 to 500 1/s. While gradually increasing the rotor speed and measuring the torque, the experiments involve microscopic observations on the time evolution of the emulsion microstructure, such as realignment, layering, breakup, and coalescence of the water droplets. The presentation will include results for light and heavy oils that have distinctly different droplet size and spatial distributions. The effects of water salinity, which alters the viscosity, as well as size, and spatial distributions of the droplets will also be demonstrated.