a1 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
In the following featured article, Boyer, Pouliquen & Guazzelli (J. Fluid Mech., this issue, vol. 686, 2011, pp 5–25) measure the normal stresses in a suspension of non-colloidal rigid spheres. They use the classical rod-climbing experiment, except that for interesting reasons the free surface near to the rotating rod does not rise but dips down. Careful techniques reveal that the normal stresses occur only above a volume concentration of 22 %. Over a period of hours the measurements drift, typical of many observations of suspensions. This is due to particles slowly migrating away from the rotating rod. A model of the migration gives good predictions of the observed changes.