Determining true material constants of viscoplastic materials from rotational rheometer data

We analyse the circular Couette flow of Herschel-Bulkley fluids to investigate the validity of the assumption that the rate of strain distributions across the gap share a common point. It is demonstrated that this is true only with fully-yielded Bingham-plastic flow. In other cases, e.g., in partially-yielded Bingham-plastic flow or fully-yielded Herschel-Bulkley flow, the common point for the fully-yielded Bingham case provides a good approximation for determining material constants only if the gap is sufficiently small. We also revisit the important issue of determining material properties of viscoplastic fluids by using "true values" for the rate of strain and demonstrate that the material properties can be very different from those obtained using "apparent' values for the rate of strain.