Surface dilatational rheology measurements for oil/water systems with viscous oils

Loss modulus determined after correction for the viscosity effects. This work presents an application of the capillary pressure tensiometry (CPT) for accurate measurements of the surface dilatational elastic and loss moduli of the interface between water and transparent oil phases with viscosities up to 10,000 mPa s. Surface rheological studies involving viscous oils are not possible with other available methods due to the considerable bulk viscous forces. Theoretical estimations show that successful measurements with such systems are possible by using a suitable frequency range of the oscillating spherical drop method by CPT. Measurements with oils having viscosities between 5 and 10,000 mPa s at a frequency smaller than 1 Hz were performed using the oil as outer phase and the aqueous surfactant solution as inner (drop) phase. As predicted by the theory the measured surface elastic modulus did not depend on the viscosity (within experimental accuracy). Three different approaches to account for the contribution of the bulk shear viscosity to the measured pressure signal were analyzed and applied. The results showed that if exact numerical corrections are used the calculated loss modulus also did not depend on the viscosities of the bulk phases. The two other methods used lead to errors, sometimes significant.