Effect of confinement on the rheology of a yield-stress fluid

We have studied the flow of Carbopol, a polymer microgel, confined to approximately square microchannels with widths ranging from 451 µm down to 54 µm. Velocity profiles in the midplane of the channels were measured using particle image velocimetry. Carbopol is a yield-stress fluid, and the velocity profiles measured in the larger channels agreed well with simulations based on its measured bulk-scale rheology. In microchannels 150 µm or smaller in width, however, the velocity profiles could not be fitted by a model with a finite yield stress, but instead were well-described by a power-law model with zero yield stress. We explain the vanishing of the yield stress in terms of the confinement by the microchannels of structural elements in the Carbopol.