The settling of spheres in a viscoplastic fluid

Results on the variation in the steady settling velocity of spheres with a lime interval between experiments in a 0.3 wt% aqueous Carbopol-941 dispersion are reported. Spheres made of stainless steel, brass, ceramics and soda lime glass ranging in nominal diameter from 1/4 to 3/8 in, were used. In each experiment, four identical spheres were permitted to settle in quick succession and the time interval between such sets was varied systematically. It was found that four spheres were sufficient to reach an asymptotic settling velocity which did not change with the addition of more spheres to the set. The dependence of the drag coefficient on a modified Reynolds number based on this asymptotic settling velocity was found to be consistent with that from prior work. The steady terminal settling velocity of the first sphere was found to decrease with increasing time intervals between experiments. The results are consistent with the hypothesis of network damage caused by shear with subsequent healing. (C) 1998 Elsevier Science B.V. All rights reserved.