Yield stress and wall effects in magnetic colloidal suspensions

The static yield stress of magnetic colloidal suspensions is measured as a function of the applied magnetic field in a controlled stress rheometer. The experimental results are analysed with the help of the magnetization curve for different volume fractions. The experimental yield stress scales as the square of the magnetization but its magnitude is always lower than the one predicted on the basis of interparticle forces. The nature of the material used for the plates of the rheometer cell, as well as the surface roughness, is shown to have a large effect on the measured yield stress.