Why teapots dribble

Pouring tea can be tricky - here's why pots dribble

Nothing ruins a tea party more than dripping tea on a pretty tablecloth.

You would think that it's possible to prevent teapots from dripping, but the fluid dynamics of the tea spout is fiendishly complicated, and it's not easy to predict or control.

In fact, the "teapot effect", describing the way tea drips from the spout in small amounts at the end of a pour, has puzzled rheologists - those who study fluids - for decades.

One fluid researcher even won an Ig Nobel Prize in 1999 for inventing a non-dripping teapot spout.

"Although this is a very common and seemingly simple effect, it is remarkably difficult to explain it exactly within the framework of fluid mechanics," says lead author Dr Bernhard Scheichl, lecturer at the Institute of Fluid Mechanics and Heat Transfer at the Technical University of Vienna, Austria.

The researchers focussed on the underside of the tea spout. When a drop of tea forms there during pouring, the edge becomes wet - and stays that way.

If the tea is pouring quickly out of the pot, the drop can be ignored.

But if the flow slows down, the under-spout drop can direct the trickle of tea, stopping it from disconnecting with the pot and forcing it to dribble down the side instead.

There are a range of complicated physical reasons for this dribble: inertia, capillary forces and viscosity all play a role.

Scheichl and colleagues believe they've cracked all the factors.

"We have now succeeded for the first time in providing a complete theoretical explanation of why this drop forms and why the underside of the edge always remains wetted," says Scheichl.

Bizarrely, the researchers believe that gravity doesn't have a large effect on the dribble of the tea, just the direction of the jet.

This means that the teapot effect would work in the same way in lower- or higher-gravity environments, although not in zero gravity.

The researchers have followed their theoretical study up with high-speed video of the teapot effect in action, but they say that their predictions need to be borne out in the real world.

"We feel an urgent need for careful and systematic laboratory experiments," they write in their paper.

This story Pouring tea can be tricky - here's why pots dribble first appeared on The Young Witness.