Yield-stress fluids are soft solids when subjected to a shear stress lower than their yield stress, but flow in response to higher stresses. I will discuss the process by which Carbopol, a model yield-stress fluid, changes from solid-like to fluid-like when it starts to flow in a vertical pipe. In a rough-walled pipe, the yielding involves a long transition with several steps: elastic deformation, the onset of wall slip, yielding at the wall, and finally a steady-state plug flow that is well-described by the predictions of the Herschel-Bulkley model. I will then discuss experiments on the flow of Carbopol confined to square microchannels with sides ranging from 500 µm down to 50 μm. In the larger channels, measured velocity profiles agree well with simulations based on the bulk rheology of the Carbopol and the Herschel-Bulkley model. In contrast, in microchannels smaller than 150 μm the velocity profiles could not be fitted by a model with a finite yield stress, but instead were described by a power-law model with zero yield stress. I will discuss the vanishing of the yield stress in terms of the effect of confinement on the microstructure of the Carbopol.
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