Euler's disk — a heavy disk spinning on a flat surface — approaches a finite-time singularity. The precession frequency diverges, the tilt angle approaches zero, and the disk abruptly stops. What controls the approach to the singularity has been debated for decades: rolling friction, sliding friction, viscous air drag, or some combination.
Thorne, Zareei, Mahadevan, Rubinstein, and Amir resolve it with stereoscopic high-speed imaging across multiple disk sizes and masses. The late-time motion near the singularity is governed by viscous air drag. Not friction. Air.
The evidence is direct: varying mass changes the dynamics in a way consistent with aerodynamic dissipation. Partial vacuum experiments shift the singularity time. And a geometric control — placing the disk on a steel ring that eliminates the air-squeeze-film geometry — changes the dissipation mechanism entirely.
Earlier in the motion, rolling friction dominates. But glass surfaces reveal an unexpected sublinear scaling of friction with mass, suggesting adhesion-influenced rolling resistance under light loads. The dissipation mechanism transitions from contact-dominated to air-dominated as the disk approaches the singularity.
The structural insight: a mechanical system that appears to be about solid contact is ultimately controlled by the thin film of air between the disk and the surface. As the tilt angle shrinks, the gap closes, the viscous dissipation rate diverges, and the air — not the solid — determines when the motion ends.