A Comparison of Models of Linear Collisions Between Spherical Particles in the Pendular Regime

This study examines granular collisions between spherical particles coated with a thin layer of viscous liquid, such that the capillary number is high and viscous forces dominate. Measurements were performed using high-speed particle tracking velocimetry. The measurements were used to examine four theoretical models that describe the collisions. The viscous force scales with the inverse of the separation distance between particles. The models limit the viscous force by setting a minimum approach distance. A fair match between experiments and simulations was obtained for all models by fitting this minimum approach distance. However, in some cases, to achieve a good match to the experiments, the minimum approach distance had to be set to a non-physical value. The best, and most physically reasonable, agreement was obtained when the viscous force accounted for the curvature of the surface and the minimum approach distance was determined using an empirically-fitted glass transition pressure model.