A comparison of pendulum experiments and discrete-element simulations of oblique collisions of wet spheres

Oblique collisions of two spherical particles coated with a thin layer of viscous liquid are considered. Experimental measurements are performed using particle tracking velocimetry and the results are compared to viscous force and capillary force models via numerical simulation using the discrete-element method. Comprehensive experimental data for collisions with an impact angle between 0 & DEG; and 60 & DEG; are presented to ensure future models can be rigorously validated. Collisions are characterized by the normal Stokes' number (a dimensionless ratio of normal inertial forces to normal viscous forces), and the normal coefficient of restitution (a dimensionless ratio of postcollisional normal velocity to precollisional normal velocity). Good agreement was found between the models and the experiments at high Stokes' number, where the models are dominated by the normal components. As the tangential forces become more significant (i.e., at low to medium Stokes' number, and high collision angle), agreement between the simulations and experiments is poorer. Furthermore, the models predict a decreasing rotational velocity with increasing Stokes' number past some critical Stokes' number. A coefficient of friction term was implemented and found to improve agreement for the rotational velocity postcollision.