Physicists say they’ve finally solved the teapot effect—for real this time

Dropping below the critical flow rate results in the wetting of the edge, and the telltale dribble of the teapot effect.

The dribbling of tea down the side of a teapot while pouring—known as the teapot effect—is a minor annoyance for regular tea drinkers. But for physicists around the world, it has posed a knotty theoretical problem spanning decades of research, garnering an Ig Nobel Prize along the way. We thought researchers had finally closed the book on the mystery of the teapot effect in 2019, when Dutch physicists came up with a quantitative model to accurately predict the precise flow rate for how much (or how little) a teapot will dribble as it pours.

But apparently there was still more work to be done to fill a few holes in the theory, and physicists at the Vienna University of Technology (TU-Wien) and University College London took them on. The researchers say they’ve finally developed a complete theoretical description for the teapot effect that captures the complex interplay of inertial, viscous, and capillary forces that collectively serve to redirect the flow of liquid when certain conditions are met. Gravity, however, proved to be less relevant; all it does is determine the flow’s direction. That means you’d still get the teapot effect on the Moon, according to the authors, but not if you poured tea on board the International Space Station.

The researchers presented their theoretical calculations in a paper published in the September issue of the Journal of Fluid Mechanics. And now they have announced the results of experiments they conducted to test their theoretical model. Spoiler alert: the model passed with flying colors. And while it might seem to be a trivial conundrum, the insights gained could help us better control fluid flow in, say, microfluidic devices.

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