Properties of carbopol solutions as models for yield-stress fluids

Abstract

The effect of pH on the non-Newtonian viscosity (η) of aqueous Carbopol 940 solutions is presented with high resolution (pH increments about 0.4) between pH = 2.6 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 (γ) over the range γ· = 8 to 195 s-1. Yield stresses and shear-thinning η(γ) 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 η(γ) to the dynamic viscometric properties η′(ω) and η*(ω) for comparable ranges of γ· and frequency (ω): A fairly close match was found between η and η* but η ≫ η′. 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.