Experimental collisions of varying roughness wetted particles in the pendular regime compared to numerical simulations

In this work rectilinear collisions of spheres coated in a thin viscous liquid film are considered, where the surface roughness of the spheres was varied. Experiments were performed using a Newton's cradle apparatus and the collision dynamics was measured using particle tracking velocimetry. The experiments showed that the dry and wet coefficient of restitution decreases as the roughness increases. Experimental collisions are compared with numerical simulations to examine criteria that limit the viscous force. We show that a model in which the liquid undergoes a glass transition is in excellent agreement with experimental measurements for smooth spheres, i.e., when the roughness of the spheres is less than the glass transition length. For rough particles, a constant minimum separation distance is more accurate than the glass transition model, which is consistent with the idea that contact occurs on the roughness elements. Furthermore, smoothed particle hydrodynamics (SPH) simulations were used to examine the viscous flow in detail. The SPH simulations accurately predicted the collision outcome for smooth spheres and showed that the maximum pressure was greater than the glass transition pressure used for the discrete element method simulations, supporting the feasibility of the glass transition model. The SPH simulations of rough particles indicate that during a collision the interstitial liquid flows through microchannels between roughness elements as a mechanism to alleviate pressure buildup, and reduce the viscous force consistent with the experimental observations.