Hydrodynamic interactions between two spheres at contact

We develop a model of contact interactions between two spherical particles immersed in a viscous fluid, under a very low Reynolds number. This model a llows us to interpret results of our experiment, in which the settling moti on of a ball in the vicinity of another fixed sphere is accurately measured with laser interferometry. Due to the symmetry of the experimental setup a nd the reversibility of the Stokes equations describing the fluid flow, the trajectory and velocity of the moving sphere center are expected to exhibi t symmetry with respect to reflection in the horizontal plane containing th e fixed sphere center. However, no such symmetry is observed if the particl es "touch" each other. Our model accounts for symmetry breaking by the cont act friction between surfaces-such a force appears only when the moving sph ere center is above the horizontal plane containing the center of the fixed particle. The model predicts two intervals of motion, at contact: pure rol ling and rolling with slip. The existence of both types of motion, with a s harp transition from one to another, has been verified experimentally. [S10 03-651X(99)05303-9].