Effect of geometrical confinement on the flow of soft microgel particle pastes

In this paper, we determine the effect of confinement on the shear flow of concentrated soft microgel particle suspensions. Utilizing a flexure-based microgap rheometer (FMR), aqueous suspensions of swollen P(NIPAM-co-AA) microgel particles of 4μ m diameter at different volume fractions larger than 64 vol% are subjected to shearing deformations between plates separated by gaps of 5–120μ m while monitoring the stress response at different rates. Describing the stress evolution from a balance of elastohydrodynamic lubrication and elastic forces following the approach of Cloitre and Bonnecaze, it could be shown that already below a critical confinement level on the order of 10 particle diameters an increase of the interparticle pressure leads to a rising shear stress at a given rate in the yielded regime. This effect is strongest for particle volume fractions close to the critical closest packing, with two orders of magnitude increase in flow resistance when approaching the single-particle confinement level, whereas this confinement effect on the flow decreases when raising the concentration. Furthermore, it could be shown that, at the onset of the yielding of the suspensions, the confinement is increasing the effective modulus, but not the yield strain.