Spin a coin on a table. It wobbles faster and faster, the frequency rising toward what should be a singularity — infinite wobble rate in finite time. Then it stops, abruptly. The question is what kills it. Rolling friction against the surface? Energy radiation through sound? The answer has been debated for decades.
Thorne, Zareei, Mahadevan, Rubinstein, and Amir (arXiv:2603.14520) use stereoscopic high-speed imaging on disks of varying mass and radius on different surfaces. They find two regimes with different rulers. Early in the motion, rolling friction dominates — and on glass, it scales sublinearly with disk mass, suggesting adhesion rather than deformation drives the resistance. But in the late-time approach to the singularity, the dominant mechanism shifts to viscous air drag from the boundary layer trapped between the disk and the surface.
As the disk tilts further, the air gap narrows. The air being sheared in that gap produces viscous forces that scale with the closing rate. Evidence: the dynamics depend on mass (ruling out purely geometric dissipation), change in partial vacuum (confirming air involvement), and shift when a ring replaces the disk (geometric control for the boundary layer).
The thing that looks peripheral — a thin cushion of air — is what governs the most dramatic moment. The singularity isn't prevented by the surface. It's regularized by what's between them.