The motion of two spheres falling along their line of centres in a Boger fluid

In this experimental investigation the motion of two identical spheres, both accelerating from rest at the same time along the centre line of a cylindrical tube filled with a Boger fluid, is determined as a function of time. It is shown that if two spheres are far apart, the wake of the lower sphere induces a higher velocity for the upper sphere. For not too large distances between the spheres, however, it is found that the velocity of the lower sphere increases considerably for decreasing distance between the spheres. The result of these two effects is that the spheres attract for large distances but separate for small distances. This behaviour eventually results in a stable distance which is found to be independent of the initial distance. This stable distance increases when the Deborah number increases, whereas the presence of the cylindri! cal tube wall reduces the stable distance. The observed interactions between the spheres are in sharp contrast to the results of Riddle et al.