III. Collisions of small uniform spheres

Two-body interactions of small rigid and deformable spheres in dilute suspensions undergoing Poiseuille flow at Reynolds numbers less than 10-3 were studied and found to be similar to those previously observed in Couette flow. Two-body collisions between rigid spheres were symmetrical and reversible, the paths of approach and recession being curvilinear and mirror images of one another, except near the wall. The measured collision frequency agreed well with a theory based on rectilinear approach and recession, whereas the measured steady-state number of doublets was twice that predicted by the theory. The discrepancy was in part due to the existence of non-separating doublets, the orbits of which were also studied. In contrast, collisions between liquid drops were unsymmetrical, thus providing a mechanism for net migration of drops towards the tube axis in addition to the axial migration previously observed with single deformed drops.