When making shear rheological measurements on structured liquids, in particular suspensions, emulsions or foams there is a high probability that the measurement may be affected by a phenomenon known as 'wall slip'. Wall slip generally results from a local depletion of the dispersed phase near the geometry walls, effectively forming a lubrication layer at the surface. As a consequence bulk rheological properties are no longer being accurately measured leading to an underestimation of the true viscosity. A similar effect can be observed when measuring solid like materials where there is insufficient friction between the sample and the wall to support the applied stress.
Figure 1: Serrated parallel plates used to minimize slip
Wall slip can be counteracted in a number of ways when testing on a rotational rheometer, most notably by use of roughened or serrated geometries which effectively take the geometry motion into the bulk of the sample and thus maximize sample-sample contact at the expense of sample-wall interactions. For cup and bob systems vanes and splined geometries can be utilized also.
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