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 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.
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 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.
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.