The nature of particle contacts in sedimentation

Abstract

An experimental study of the contact of a falling sphere with a neutrally buoyant sphere in a viscous fluid is described. The experiments used a mixture of polyalklene glycol and tetrabromoethane as the Newtonian fluid, together with a pair of equal-sized Teflon■ and nylon spheres of 6.355 mm diameter. The spheres have microscopic surface roughness, which allows them to make physical contact when in close approach. This contact breaks the symmetry of the relative trajectory of the two spheres and affects the rate at which the heavy sphere moves past the neutrally buoyant one. The experimental observations verify the roll/slip model of Davis [Phys. Fluids A 4, 2607 (1992)] for the interaction of the two spheres in contact. This model assumes that contact prevents the nominal surfaces from approaching closer than a minimum separation equal to the effective roughness height, and that the tangential component of the contact force is described by solid friction theory. A friction coefficient of 0.28±0.02 provides the best agreement between theory and experiment, with some variation observed when the experiments were repeated for the same and different pairs of spheres.