The use of opposed nozzles configuration in the measurements of the extensional rheological properties of emulsions

Abstract

The opposed nozzles configuration was used to measure the extensional viscosities of water-in-oil emulsions of varying drop volume fractions between 30% and 80%. For the more dilute emulsions, the extensional viscosities decrease with increasing elongational rate which mimics their shear viscosity behavior. The elongational viscosities are approximately three times the shear viscosity values in accord with Trouton’s rule. For the more concentrated emulsions, where close packing of the drops occurs and where yield stress behavior begins, the apparent extensional viscosity values display a dependence on the nozzle diameter and nozzle separation. This is in sharp contrast to experiments on a high viscosity, Newtonian oil at the same torque levels which showed no nozzle size dependence. The dependence on nozzle size appears to be caused by forces on the nozzle arm arising from the yield stress of the emulsions. To demonstrate this behavior, experiments on a microgel dispersion with a high yield stress produced torque signals that increased with time and never reached steady state. We conclude that measurements of elongational viscosities of fluids with significant yield stresses are not possible with the present opposed nozzle apparatus.