Slippage during the flow of emulsions in rheometers

The steady shear and oscillatory data obtained for concentrated oil-in-water (O/W) and water-in-oil (W/O) emulsions from measuring geometries of different design and size were analyzed and compared. The measuring geometries used were: smooth cone-and-plate geometry with a cone angle of 1 degrees, smooth cone-and-plate geometry with a cone angle of 4 degrees, and serrated parallel plate geometry with a gap width of 1 mm. A controlled stress rheometer (Bohlin CS-50) was used to collect the rheological data. In the high shear stress range, the steady shear viscosity data obtained from the three measuring geometries (for the same emulsions) show good agreement with each other. However, in the low shear stress range, the viscosities measured by the smooth cone-and-plate geometries are much lower (by a factor of 25 000 in some cases) than the viscosities measured by the serrated plate geometry due to slippage at the fluid/wall interface. The viscosity-versus-shear stress plot (flow curve) obtained from the smooth cone-and-plate geometries shows a sudden break from the true flow curve with an unexpected Newtonian plateau when the shear stress is decreased below a certain value. The oscillatory data are also significantly influenced by slip effects. The stress sweep experiments show a linear viscoelastic region extended over a much wider range of shear stress when a serrated plate geometry is used. The storage modulus versus frequency data obtained from the serrated plate geometry generally fall above those obtained from the smooth cone-and-plate geometries. For some emulsions, the serrated plate geometry shows a solid-like behaviour (constant storage modulus, independent of frequency) whereas the smooth cone-and-plate geometries show a fluid-like behaviour, that is, the storage modulus falls off rapidly when the frequency is decreased. (C) 2000 Elsevier Science B.V. All rights reserved.