Simultaneous and sequential collisions of three wetted spheres

Rectilinear collisions of three wetted spheres are considered under conditions of high capillary numbers, for which viscous lubrication forces dominate over capillary forces. The viscous forces resist the relative motion, as characterized by the Stokes number (a dimensionless ratio of particle inertia and viscous forces). At high Stokes numbers, the particles penetrate the fluid layers between them with sufficient inertia that they collide and rebound. Both simultaneous and sequential collisions are simulated, and various outcomes are demonstrated: full agglomeration of the three spheres at low Stokes numbers, full separation or Newton's cradle at large Stokes numbers and even reverse Newton's cradle at intermediate Stokes numbers when there is a thicker combined fluid layer between the two target spheres than between the striker sphere and the first target sphere. When there is an initial air gap between the two target spheres, even more exotic outcomes are predicted, such as full separation after the initial collisions followed by full agglomeration or reverse Newton's cradle (intermediate Stokes numbers) or Newton's cradle (large Stokes numbers) after the subsequent collisions when the striker sphere catches back up to the target spheres. The approach and findings of this work are expected to provide input and guidance to future work on discrete-element modelling of collisions of many wet particles.