Surface-roughness–driven segregation in a granular slurry under shear

“Shear-induced segregation” (SIS) means that different species of particles in a granular material or a concentrated suspension tend to unmix under flow. In principle, any kind of difference (size, density, shape and even surface state) may lead to SIS. We report the first direct experimental evidence of SIS between spheres which only differ by surface characteristics. The effect is observed with a “slurry” made of solid spheres immersed in a viscous fluid, inside a parallel-plate shear device. We show that: i) A mixture of smooth and rough spheres of same size shows considerable SIS. ii) Rough spheres (radius aR) behave similarly to smooth but larger spheres (radius $a_S > a_R )$. iii) Segregation cancels out for a particular value of the size ratio, ${a_S}/{a_R < 1}$. These findings can be qualitatively understood from available theories about the role of surface roughness on two-sphere interactions in Stokes regime.