We characterize microfluidic flows of jammed suspensions of soft microgels (Carbopol) behaving as yield-stress fluids. We quantify the wall slip friction, i.e., the slip velocity $V$ versus the tangential stress at the wall ${\sigma }_w$. We demonstrate a transition in slip regimes, from a nonlinear behavior ($V\propto {\sigma }_w^2$) to a linear one, as the stress at the wall is increased, as expected from scaling arguments. Using fluorescent imaging to characterize the microgel size, we rationalize the two friction regimes for various samples by estimating viscous and elastic forces at the scale of the microgel particle. Only local arguments are thus necessary to predict wall slip friction, in contrast to other complex flow features such as fluidity or shear banding where bulk and surface properties appear to be strongly related.

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• Microfluidics
• Rheology
• Wall slip
• Microgels