THE NATURE OF PARTICLE CONTACTS IN SEDIMENTATION

An experimental study of the contact of a falling sphere with a neutra lly buoyant sphere in a viscous fluid is described. The experiments us ed a mixture of polyalklene glycol and tetrabromoethane as the Newtoni an fluid, together with a pair of equal-sized Teflon(R) and nylon sphe res of 6.355 mm diameter. The spheres have microscopic surface roughne ss, which allows them to make physical contact when in close approach. This contact breaks the symmetry of the relative trajectory of the tw o spheres and affects the rate at which the heavy sphere moves past th e neutrally buoyant one. The experimental observations verify the roll /slip model of Davis [Phys. Fluids A 4, 2607 (1992)] for the interacti on of the two spheres in contact. This model assumes that contact prev ents the nominal surfaces from approaching closer than a minimum separ ation 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 b etween theory and experiment, with some variation observed when the ex periments were repeated for the same and different pairs of spheres. ( C) 1996 American Institute of Physics.