The teapot effect — that tendency for liquid to dribble down the outside of the spout when pouring — is a frustration to many tea drinkers. Unraveling the fluid dynamics of this phenomenon has taken various researchers decades, but a team now believe they’ve captured the problem fully. Their full mathematical description is quite dense, but it boils down to a subtle interplay of capillary, viscous, and inertial forces.
Essentially, they found that droplets will always form just under the lip of the spout, thereby keeping that area wetted. The flow rate of the pour (along with the geometry and surface characteristics of the spout) determines how large those droplets can grow. At low flow rates, the droplets can grow large enough to redirect the entire stream around the spout’s edge, creating a hugely frustrating mess. You can see this flow rate effect in the high-speed video below. (Image credit: S. Ferrari; video and research credit: B. Scheichl et al.; via Ars Technica; submitted by Kam-Yung Soh)