Why do teapots drip?

Yum Cha

Visit any tea house in Hong Kong and you’ll see people tapping the table with three fingers. The story goes that a Chinese emperor wanted to see how normal people lived so he dressed in ordinary clothes and went to the teahouse. As he watched people gracefully pouring tea for each other without spilling a drop, he decided to try for himself and poured tea for his servants. Not wanting to give away the emperor’s true identity by bowing their thanks, the servants bowed surreptitiously with their fingers and unknowingly started a ritual that continues to this day.

Unfortunately, the tale doesn’t say whether the emperor managed to pour his servants’ tea without spilling anything, but why teapots drip is a problem scientists have been trying to figure out for a long time.

In the lab

Scientists first thought the ‘teapot effect’ must be due to surface tension but, in 1956, Markus Reiner performed an experiment which seemed to show this wasn't true. He placed a conical flask upside down in water and poured salty water, which sinks in normal water because it’s denser, over the top of it. Since surface tension has no effect underwater the salty water stream should have fallen straight down from the edge of the flask’s bottom, but Reiner found that the water followed the incline of the flask for a short while. He also established that flow rate was important. The slower the pour, the more likely the teapot was to drip. With a fast pour the tea has less of a chance to slow down, change direction and drip down the spout.

Next up to try and figure out why teapots drip was Joseph Keller, Professor of Mathematics and Mechanical Engineering at Stanford University and twice winner of the IgNobels. He reasoned it must be a combination of Bernoulli’s Principle and air pressure pushing on the water that caused it to run down the underside of the spout.

And more recently, in 2009, French scientists discovered another part of the puzzle – the effect of wettability.

Wettability is a measure of how much a liquid likes being in contact with a surface. At one end of the wettability scale are materials like clean glass, where water tends to spread out. At the other end are superhydrophobic materials. They occur naturally such as the lotus leaf, or, thanks to recent inventions, can be applied as a spray, making any material impervious to liquids or dirt.

The French scientists investigated what effect wettability had on dripping teapots. "In our experiment we varied systematically the wettability of a kind of spout as well as the geometry, particularly the radius of curvature, of the spout. What we found was to avoid dripping and to maximise the ejection of water, it's much better to use a hydrophobic coating," says Lyderic Bocquet, one of the scientists behind the research.

The wettability of a surface determines how a liquid behaves. Liquids tend to stick slightly to solids,egturning a page with a wet finger, and it’s this which causes a liquid to spread out over a surface. However, the molecules in a liquid are also attracted to each other, resulting in the phenomenon of surface tension, and it’s this which causes the liquid on a surface to form small droplets.

Superhydrophobic materials usually have a highly textured surface at very small scales (nano range) making it hard for liquids to stick to the surface and spread out. Instead, the internal attractive forces are dominant causing the liquid to ball up as drops.

So a superhydrophobic teapot spout prevents dripping by breaking the stream of tea into separate little droplets which fall in to your cup rather than stick to the underside of the spout.

Bocquet and his colleagues also looked at how the curvature of a spout affects dripping. They found that if the spout is thin and has a small radius it is less likely to drip because it’s harder for the flow to make the sharp turn onto the underside of the spout.

But for all the research scientists have done, whether a teapot drips is sometimes in the hands of the teapot maker.

Teapot maker

"Teapots are almost the pinnacle of learning how to throw clay on a wheel," says Jess Joslin, a potter. "There are so many different stages involved: the body, the spout, the lid and the handle. You make these individually and assemble them over a couple of days. This can take at least ten days or a month if you are making a batch. And then maybe it dribbles."

So for most, dripping teapots are just a fact of life. But not for the housewives of the 1920s who knew how to change the wettability of surfaces and get rid of drips – by rubbing butter on the underside of the spout

Further reading

How kettles whistle

Top image: Flickr

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