Drop a dry bead onto a surface and it bounces. The coefficient of restitution — the ratio of rebound speed to impact speed — is a material property: roughly constant for a given bead-surface pair. A steel ball on glass might return 0.9 of its impact velocity. The physics is elastic strain plus some plastic dissipation. Simple.
Wet the bead. Now the coefficient of restitution depends on two numbers: the Stokes number (impact inertia relative to viscous drag) and the dimensionless film thickness (liquid layer relative to bead diameter). Singh, Windows-Yule, and Nair (arXiv:2603.23100) use smoothed particle hydrodynamics simulations to map the relationship, and they find two distinct regimes with different power-law scaling.
This is not a smooth transition. The energy dissipation mechanism changes. In one regime, the liquid film absorbs energy through viscous squeeze-flow — the bead pushes liquid out of the narrowing gap between surfaces, and the viscous resistance does the dissipating. In the other, the film dynamics change character — perhaps because the film is thin enough that lubrication approximations break down, or thick enough that bulk flow dominates.
The structural lesson: a liquid film is not just a damper bolted onto an elastic collision. It changes which physics governs the collision. The dry collision's restitution is a property of the materials. The wet collision's restitution is a property of the flow. Same bead, same surface, but the addition of a thin liquid layer moves the problem from solid mechanics to fluid mechanics — and the scaling laws change accordingly.
Granular systems in industrial processing, pharmaceutical manufacturing, and geological flows are almost always partially wet. The assumption that wetting “reduces” the coefficient of restitution — that it adds dissipation to an otherwise dry problem — misses the regime transition. It's not more damping. It's different physics.