Rheology of microgels in single particle confinement

In this work, we investigate the shear rheology of Carbopol 981 microgel particle suspensions, confined between shearing plates with gap separations from 5 to 100 mu m. We show that even for confining gaps smaller than that of the gel particle size, the yielding of concentrated microgel suspensions is delayed to stress levels above the bulk yield stress. Furthermore, for stresses below this new yield point, slip is described by elastohydrodynamic lubrication theory as long as the direct confinement of the single gel particles between the shearing surfaces is limited to a Hertzian deformation. For a strong, non-Hertzian particle deformation, the slip layer breaks down and leads to a frictional interaction of the single confined particle with the two shearing surfaces, depending on their surface roughness. Lubrication pressures and friction coefficients have been quantified with in situ normal force measurements on the confined particles, which have also been utilized to unambiguously determine the relevant swollen particle dimensions.