Particle motions in sheared suspensions: XXXI. Rotations of rigid and flexible dumbbells (experimental)

Abstract

The rotations of models of rigid and flexible dumbbells suspended in viscous oils were studied in Couette flow. Rigid dumbbells of fixed interparticle distance r were made using 1- to 3-mm-diameter spheres, rigidly coupled or freely rotating, joined by rigid 38-μm-diameter filaments or 100-μm-diameter metal wire. Flexible dumbbells of variable r were made using elastomer filaments of 56-μm-diameter. The translational and rotational motions of the spheres as well as that of the dumbbell axis were recorded on videotape or cine film, and the results compared with theory assuming negligible hydrodynamic resistance of the connecting filament or wire. As predicted, it was observed that, at a given r, the measured dimensionless period of rotation TG of the dumbbells of freely rotating spheres was appreciably greater than that of rigidly coupled spheres. When, due to viscous damping, the sphere rotations were hindered, TG was significantly reduced. The measured dumbbell and sphere angular velocities were in good agreement with theory for dumbbells exhibiting free or hindered sphere rotation. Flexible dumbbells whose spheres and filament were initially positioned in the equatorial plane, adopted a limiting orbit (limit cycle) characterized by a periodically varying r, appreciable bending of the filament, and an increase in TG with increasing G. The measured sphere paths and TG were in fair agreement with theory, although there were significant discrepancies due to the rigidity and hydrodynamic resistance of the filament, especially at low G. The results are discussed in the light of the previously described flow behavior of primary energy and secondary energy minimum doublets of latex spheres, as well as of polymer bridged doublets.