Anomalous Wall Effects in Parallel Plate Torsional Flow of Highly Concentrated Emulsions

The rheological behaviour of a highly concentrated oil-in-water emulsion (75.1% by volume oil) was studied employing parallel plate torsional flows, with main emphasis on the wall effects. The steady shear and oscillatory shear data were collected with a controlled stress rheometer using smooth and serrated parallel plate geometries. The effect of the gap-height (between the parallel plates) on the rheological properties was also determined. When a serrated parallel plate geometry is used, the wall (slip) effects are found to be negligible. The data obtained by employing various gap heights overlap with each other. However, in a smooth parallel plate geometry, the emulsion behaviour is strongly affected by slip at the wall. The data obtained by employing various gap heights are different. In the absence of wall effects, the emulsion exhibits a highly nonlinear behaviour characterized by yield stress and high values of storage modulus.