Rheology of water in crude oil emulsions

The present paper deals with the rheological study of simplified sea water in crude oil emulsions. This includes viscosity dependence vs. shear rate, creep experiments and oscillatory shear measurements. Our results show the existence of a critical water volume fraction phi(c), in addition to the maximum packing fraction phi(m),. This critical fraction marks the onset of physical contact between water droplets. The determined thickness of the hydrodynamic layer due to surfactant molecules is shown to decrease as the water volume fraction increases. On the other hand, when emulsions contain an additional surfactant, the loss modulus G " measured at f= 1 Hz shows a maximum for a given shear strain gamma(max). Such shear strain, however, decreases as the water volume fraction increases and remains constant above phi similar to phi(c). Finally, we attempt to scale shear rate values with relaxation time determined from creep experiments using the analogy with concentrated macromolecular solutions. (C) 1998 Elsevier Science B.V. All rights reserved.