The teapot effect
Why does liquid poured slowly from a spout often dribble down the outside of the spout instead of falling cleanly into the cup?
History and references
Settled in 2010 by Cyril Duez, Christophe Ybert, Christophe Clanet, Lydéric Bocquet (Universite de Lyon).
Physicist Markus Reiner named the 'teapot effect' in 1956 while studying non-Newtonian fluids, but a first-principles account of why slow pours dribble down a spout remained elusive for decades and was treated as a minor curiosity of fluid dynamics. In 2010 a Lyon group led by Lydéric Bocquet showed experimentally and theoretically that the effect is a 'hydro-capillary' phenomenon: whether the flow separates cleanly from the lip depends on a balance between inertia, gravity, and the wetting properties of the spout surface. Coating a spout to be more hydrophobic, or sharpening its edge, suppresses dribbling, confirming the mechanism and enabling practical no-drip spout designs.
A kitchen annoyance turns out to hinge on the same wetting physics used to design non-stick coatings.
Explanations that circulated
- That the dribble is purely a low-flow-speed/gravity effect fixable by 'just pouring faster', ignoring that surface wettability of the spout material is the controlling parameter.
- Explanations invoking surface tension alone, without accounting for the hydro-capillary balance between inertia, wetting (contact angle) and the curvature of the lip that the 2010 study identified as decisive.
References
- Wetting Controls Separation of Inertial Flows from Solid Surfaces, C. Duez, C. Ybert, C. Clanet, L. Bocquet, Physical Review Letters, 2010
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