Written by Raymond Nakamura
Once upon a time (before the Internet), Raymond earned his doctorate studying the hydrodynamics of sand dollars. Nowadays he rents his brain at raymondsbrain.com for writing, cartooning, and thinking, when he is not washing the dishes, walking the dog, or helping his daughter with homework. Follow him on Twitter @raymondsbrain.


different explanations for the teapot effect

Created date

Sunday, October 18, 2015 - 8:00am

Ever Wonder About the Teapot Effect?

During dinner at a Chinese restaurant, I made a mess pouring tea. I wondered whether I might find some science to avoid dribbling down the spout onto the tablecloth. 

What's Going On?

This dribbling, aka the Teapot Effect, is steeped with a tempest of explanations. Back in the 50s, Markus Reiner took the first turn, suggesting internal vortices in the fluid kept the fluid against the teapot. Soon after, Joseph Keller pushed for atmospheric pressure as the main culprit. Later, Keller and colleague Jean-Marc Vanden-Broeck expanded the pressure theory with equations and everything, which won them an Ig Nobel prize in 1999. Flying Circus of Physics guru Jearl Walker also upheld the atmospheric pressure idea in his Scientific American article in 1984, which included a number of experiments and observations.

If theory is not your cup of tea, here are some concrete ways to avoid the teapot effect.

Pour Sport

Pour quickly, so the momentum of the tea can overcome other forces acting on it. If poured slowly, the tea is more likely to dribble. This may also be more likely if the pot is too full.

Keep Sharp

A thin, sharp-edged lower lip makes it more difficult for the fluid to flow around the lip of the spout. For ceramic teapots, a problem can be this construction makes them more vulnerable to breakage. Metal teapots can have an advantage here. 

Narrow Spout

Narrow spouts seem to work better. Maybe the fluid has less air contact as it's coming out. Conversely, this is why it can be difficult to pour something from a wide-mouthed container like a drinking glass.

Keep It Up

If the lower lip of the spout heads upward in the pouring position, this makes extra work for the fluid to get around the edge. I was in a teashop recently, asking about the least drippy teapots and all the pots they used to serve tea samples had this feature.

Superhydrophobia

Superhydrophobic materials don't just dislike water, they push it away. Those French scientists added superhydrophobic material to spouts and showed it eliminates the teapot effect. They even controlled the effect electronically. One of things they added was soot and I don't think that would be great for the tea. But maybe the next time I go for dim sum, I could try some pork fat.
 

If you have any explanations or observations on the Teapot Effect, short and stout or otherwise, pour them out in the comments.
 

Comments

On my tour to China, I had

On my tour to China, I had visited a pottery which crafted air tight tea pots (some tea on the lid help complete the seal). Each had a small hole at the top that could be covered by the thumb. When pouring the tea, you wold release the hole for the tea to flow out. When covering the hole, the tea would stop abruptly. No dripping. I almost bought one of them (~$40CAD), but the pots were too small for me and I didn't want to handle fragile items on the flight back.

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