Rheological measurements were performed on aqueous dispersions of two commercial crosslinked polymer microgels, Carbopol Ultrez 10 and Carbopol ETD 2050, prepared over a wide range of concentration and pH. For all concentrations studied, both the yield stress and the elastic modulus initially increased dramatically with pH and displayed broad peaks at intermediate pH. This is consistent with the onset of jamming of the Carbopol particles due to a rapid increase in particle size caused by osmotic swelling in the presence of NaOH. Scaling of both yield stress and elasticity with concentration was observed only at higher concentrations, which we believe indicates a change from a percolated structure at low volume fractions to a space filling network of compressed particles at high volume fractions. This model is supported by confocal microscopy of fluorescently dyed Carbopol dispersions.

We report the results of an experimental study of the microstructure of dispersions of Carbopol ETD 2050, a model yield-stress fluid. Using two different light scattering instruments, measurements were made over three decades in scattering wave vector, from 0.02 to 25 μm⁻¹. These measurements reveal microstructure characterized by two length scales: a longer length scale, 6 μm and larger, that depends on Carbopol concentration and the pH of the dispersion and a shorter length scale of approximately 400 nm that is independent of both sample concentration and pH. We relate these results to shear rheology measurement of the yield stress of these materials.