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.
Markus Reiner first described the teapot effect in 1956 and went on to help pioneer the field of rheology (the study of how liquid flows). As I’ve written previously, the late Stanford engineer and mathematician Joseph B. Keller conducted his own investigation and concluded that the dripping was due to air pressure rather than surface tension, as many had assumed. He and a colleague, Jean-Marc Vanden‐Broeck, published a paper in 1986—work that earned them an Ig Nobel prize in 1999. According to Keller, the pressure of the liquid is lower at the pouring lip than in the surrounding air, and the latter pushes the tea against the lip and outside of the spout.
Here’s the basic working explanation. At higher flow rates, the layer of fluid that is closest to the teapot’s spout detaches so it flows smoothly and doesn’t drip. At lower flow rates, when flow separation occurs, the layer reattaches to the spout’s surface, resulting in a dribbling flow. The spout’s diameter, the curvature of the lip, and the “wettability” (the preference of a liquid to be in contact with a solid surrounded by another fluid) of whatever material the teapot is made of are also factors that can affect whether or not the kettle drips.
But they aren’t the primary culprit. In 2010, French physicists demonstrated that the actual cause of the dribbling is a kind of “hydro-capillary effect” that prevents (at slower pour speeds) the liquid from detaching from the spout for a smooth, clean flow. All the other factors play a role in determining how strong that hydro-capillary effect will be. Those physicists suggested making the lip of the spout as thin and sharp-ended as possible to reduce the dribble, perhaps even coating the lip with ultra-water-repellant materials.