Solving Teapot Effect

'Teapot effect' solved
Beating the teapot effect with a superhydrophobic coating. Top: water flow under the spout of an (hydrophilic) teapot, exhibiting a bending of the streamlines (a), and drip-ping as the water flow decreases (a'). Bottom: In contrast, a teapot with a spout coated by a superhydrophobic coating (here black soot) fully avoids dripping (b and b'). Image: FIG 1. from arXiv:0910.3306.
( -- A team of scientists from France have worked out why teapots dribble at low flow rates, and how to stop them. The effect is called the "teapot effect", and solving it could finally put an end to tea stains from dribbling teapots.

Previous research on the teapot effect has found that at higher flow rates the layer of fluid closest to the surface of the teapot spout becomes detached from it and the fluid flows smoothly. At low flow rates there is flow separation in which the reattaches to the surface instead of flowing over it, causing the flow to stop and start; in other words to dribble. A number of factors are known to affect how much the teapot dribbles, such as the diameter of the spout, the teapot material, and speed of flow, but until now no one has come up with a scientific solution.

experts C. Duez, C. Ybert, C. Clanet, and L. Bocquet, from the University of Lyon have solved the problem by identifying the fundamental cause of dribbling: a "hydro-capillary" effect that keeps the liquid in contact with the surface as it leaves the spout. The other factors -- "wettability" of the teapot material, of the flow, and curvature of the lip -- determine the strength of this effect.

The authors suggest that one way to avoid dribbling is to make the lip of the spout as thin as possible (as in metal teapots, which rarely dribble), while the ultimate way to instantly stop dribbling and end the teapot effect for good is to use a thin, sharp-ended spout and coat the lip with one of the latest super-hydrophobic materials. These materials strongly repel water and prevent the tea from clinging to the teapot material, which adds weight to the old idea that smearing butter on the spout stops dribbling.

Duez and his colleagues also suggest that some super-hydrophobic materials can be controlled electronically, and while it is hard to see why anyone would want a teapot with a dribble switch, there may be applications in controlling fluid flow in microfluidic devices.

The research paper is published online in

More information: Beating the teapot effect, C. Duez, C. Ybert, C. Clanet, L. Bocquet, arXiv:0910.3306

© 2009

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User comments

Nov 02, 2009
I would have thought this was already known science. I learned about hydrophilic and hydrophobic substances in school, studying chromatography in chemistry class. Or am I missing something?

Nov 02, 2009
nuge: by 'solved' they probably meant from the perspective of an engineer, not just in understanding the theory but also in having successfully applied the knowledge for a real world solution. when reading science articles online you have to separate how online journalists write about something and what the something actually is.

these content writers are never going to accurately or precisely interpret the statements of researchers, so don't be surprised to hear confusing phrases on physorg.

Nov 02, 2009
A teapot engineer? Imagine telling people that at parties..

I know what you're saying, but even so I'd have thought such an effect is routinely exploited in materials engineering.

Nov 02, 2009
This is heroic research. We've all discovered on our own that as users, we have to try and keep the flow rate high and end it with the right motion.. and we occasionally fail and blame ourselves. But it's not our problem nor our area of expertise.. it's those blasted teapot designers who were content with the status quo of teapot design and failed to address their obvious weaknesses.

More seriously, this might make people pay attention to this effect in other areas. Who knows - it could be important in fuel flow and whatnot.

Nov 02, 2009
I hope they finish soon so they can switch to more important research; the dynamic heat exchange layer between cheese and sauce on pizza and how to design the interface to be more amenable to the tops of peoples mouths.

Nov 02, 2009
wow, because there arent more important issues in the world today.

Nov 02, 2009
ok, this research isn't going to win noble prizes but ever since I was a child I've been acutely aware that pouring water or milk out of jug could result in spillages if I was to slow when tilting the jug. Now I know *exactly* why this is so :-) good article

Nov 02, 2009
wow, because there arent more important issues in the world today.

It isn't all linear accelerators nad fusion research. Everything on your desk has gone through the hand of engineers. Simple things like the placement of the shift key on a keyboard or how the rings work on your three-ring folder are engineered solutions and require a lot more research than you'd think to get them 'right'.

Be grateful that there are people out there doing that nitty-gritty stuff. Without it your life would be AMAZINGLY uncomfortable.

Nov 03, 2009
For hundreds of years, teapot engineers have existed and have flourished. They are called "potters" and craft teapots and other articles from dirt. Those of us that have made teapots have known that to prevent dribbles, you have to make the spouts with a sharp edge, and the same goes for pitchers or anything else with a pouring spout that has to accommodate very low flows. So we now have physics catching up with the manual arts; I love it!

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