Shear rheology of confined complex fluids

Orateur : Christian CLASEN (Department of Chemical Engineering -Division of Soft Matter, Rheology and Technology- KU Leuven, Belgium)

When the length scale of flow confinement for a complex fluid approaches the length scales of its internal microstructure, boundary effects such as wall slip, cohesive and adhesive failures can occur and will influence the flow behavior. In this paper we present experimental observations of microgap dependent shear rheological measurements on emulsions and suspensions of soft and rigid model particles under confinement with a flexure based microgap rheometer (FMR). We will show how the general slip and yielding behaviour of the complex fluids are affected in particular beyond a direct confinement of the single micro structural elements. For soft microgels a non-Hertzian response on strong confinement leads to a slip layer decrease and eventually to a breakdown of the lubrication layer depending on the length scale of the surface roughness of the shearing plates. At confinements beyond the hydrodynamic lubrication layer breakdown, the microgels exhibit a concentration independent boundary lubrication related to the shearing surface surface roughness. Both single particle confinement stresses and boundary lubrication friction coefficients are quantified for the first time by in situ measurements of the normal force arising from single particle confinement on the micrometer level.

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