PARTICLE MOTIONS IN SHEARED SUSPENSIONS III.: FURTHER OBSERVATIONS ON COLLISIONS OF SPHERES

Abstract

Two-body interactions between glass spheres of diameters a 1 and a 2 caused by velocity gradients vary with a 1 /a 2 . When 1 < 2, well-defined collisions similar to those previously reported for spheres of equal size can be observed. Fair agreement is found between the experimentally observed and calculated collision frequencies over a range of particle concentrations and velocity gradients. When a 1 /a 2 > 2 the particles are separated at all times and the phenomena of interaction are more complex. Single air bubbles rotate at the same angular velocity as rigid spheres. When two air bubbles of equal size are brought into collision a doublet is formed; instead of the mirror-image separation observed with neutral rigid spheres, the doublet continues to rotate for as many as 60 rotations before coalescence occurs. Less frequently a doublet with distinct particle separation is observed. Periods of rotation of both types of doublet and certain details of the rotational orbit of a doublet of touching air bubbles have been measured and compared with values predicted from Jeffery's theoretical equations for rigid ellipsoids. Apart from their intrinsic interest, the phenomena described are of importance in theories of viscosity and coagulation of suspensions and colloidal dispersions.