Spheres and interactions between spheres moving at very low velocities in a yield stress fluid

The drag force on two spheres moving at very low controlled velocity in a viscoplastic fluid was studied as a function of the distance separating them. Two configurations were studied, namely two spheres with their centre lines along or perpendicular to the flow. The influence of the surface roughness of the spheres governing fluid adherence and slip at the wall was quantified. The drag force on an isolated sphere was also measured and used as a reference. Correlations for predicting the drag coefficient and stability criterion, with respect to sedimentation, are proposed. These results show that viscoplasticity reduces the extent of the interactions in comparison with the case of a Newtonian fluid.