Quantitative analysis of mixer-type rheometers using the Couette analogy

A procedure based on a Couette analogy, to quantitatively analyze torque/rotor speed data in order to extract viscosity/shear-rate curves using non-conventional geometries is presented. It is first validated using a relatively simple geometry for which the equivalent internal radius used in the analogy can be analytically obtained. The results showed that the equivalent internal radius depends only slightly on the nature of the fluid and that there is an optimal radial position r* in the analog Couette gap where the calculations can be easily performed for computing the viscosity/shear-rate data from torque/rotational speed data. The experimental results with complex geometries and complex fluids are found to coincide, within experimental errors, with those obtained using standard geometries over a wide range of shear rates. The approach is also found to be very useful to evaluate shear-rate and viscosity data in Couette viscometers when large gaps are used with non-Newtonian fluids.