Measurement of the hydrodynamic surface roughness of noncolloidal spheres

A new technique for measuring the surface roughness of noncolloidal spheres is presented. The time for a sphere initially in contact with a smooth surface to fall away under the influence of gravity through a viscous fluid is shown to be related to the largest scale of surface roughness of sufficient surface coverage to support the particle. The ratio of the time taken for a sphere to fall one particle diameter from a smooth mica plane to that for the sphere to fall between one radius and one diameter from the plane thus provides a means of measuring the effective hydrodynamic surface roughness of spheres. This technique was employed to measure the roughness of eight types of particles ranging from 43–6350 μm in diameter. The roughnesses were found to be on the order of 10−2 to 10−3 particle radii, and were in agreement with independent observations using a scanning electron microscope and an optical profilometer.