Macroscopic vs. local rheology of yield stress fluids

From MRI velocimetry we measure the local flow characteristics of a Carbopol gel in a Couette geometry under different inner cylinder rotation velocities. Associated with torque data under the same flow conditions we deduce the local, steady-state, simple shear, constitutive equation of the material within a relatively wide range of shear rates [10(-2): 100 s(-1)]. Then we show that in this range of shear rates this "local" behaviour is in excellent agreement with the "macroscopic" behaviour deduced from conventional rheometry with cone and plate and Couette geometries. We can conclude that this material effectively behaves as a simple yield stress fluid with a Constitutive equation well represented by a Herschel-Bulkley model. This behaviour. likely due to the soft-jammed structure of the fluid. contrasts with that of aggregated systems which exhibit thixotropy and shear-banding at low shear rates. (C) 2008 Elsevier B.V. All rights reserved.