Properties of carbopol solutions as models for yield-stress fluids

The effect of pH on the non-Newtonian viscosity (eta) of aqueous Carbopol 940 solutions is presented with high resolution (pH increments about 0.4) between pH = 2.8 and 12.7. Aqueous NaOH was used to adjust pH of 1.48 wt % Carbopol solutions. A Contraves viscometer was used to measure steady-flow shear stress at known shear rate (gamma) over the range (gamma)over dot = 8 to 195 s(-1). Yield stresses and shear-thinning eta(gamma) were observed. Data were fitted with a Herschel-Bulkley model, whose parameters (including yield stress) were expressed as functions of pH. Comparisons were made of eta(gamma) to the dynamic viscometric properties eta'(omega) and eta*(omega) for comparable ranges of (gamma)over dot and frequency (omega): A fairly close match was found between eta and eta* but eta much greater than eta'. pH dependence included previously unreported extrema of h(pH) in the range of pH = 6.2 to 6.6. Because of sensitive control of rheological properties with pH, Carbopol solutions can be used to mimic a great range of shear-thinning and yield-stress behavior that should make them useful for model studies directed toward process and equipment design and evaluation.