Normal stress measurement in foams and emulsions in the presence of slip

Steady shear measurements on three yield-stress fluids, a foam and two types of emulsions, were carried out using a smooth torsional parallel-plate geometry, with different gap spacings to assess the effect of wall slip on the normal stress differences. The classical Mooney analysis gave slip-corrected flow curves that agreed with data from a roughened cone-and-plate for all three materials, despite the fact that the compatibility requirement between the flow curve and slip velocity functionalities was not satisfied, indicating that the method is robust for this geometry. All three materials exhibited finite normal stresses following yielding, with power-law exponents of approximately 0.4, as well as shear stresses that can be represented by the Herschel–Bulkley model. The systems showed complete slip for shear stresses below the yield stress, with the stress transmitted to the unyielded material through a thin liquid layer causing an elastic deformation that was measurable for the unyielded foam, which can be represented by the Mooney–Rivlin elastic constitutive equation. The unyielded foam exhibited large sample-to-sample variations in the apparent modulus for identical loading conditions.