Sing along with me — ‘I’m a little teapot, short and stout, and now scientists finally understand why I’m always dripping from my spout.’
Many a spotless tablecloth is cursed by the “teapot effect”, which sees liquid drip down the sides of the pot if it’s poured too slow, instead of forming a separate flow.
Since Markus Reiner’s 1965 description of the phenomenon, physicists have studied it. Reiner was the pioneer in rheology (the study of moving matter).
But only now have a team led from the Vienna University of Technology (TU Wien) managed to develop a complete theoretical understanding of why the effect occurs.
The key, they explained, lies in how a drop forms on the underside of the edge of the spout — one whose size is dependant on the speed at which tea is poured.
When the speed drops below a certain threshold, it becomes too large for the water to flow all the way around the edge. The pot will then dribble downwards.
Researchers also captured tea flowing at different speeds with high speed cameras in order to replicate the effect.
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Sing along with me — ‘I’m a little teapot, short and stout, and now scientists finally understand why I’m always dripping from my spout.’ Pictured here: Tea is best when it is made quickly.
The key, researchers have found, lies in how a drop forms on the underside of the edge of the spout that can redirect the flow if large enough — and whose size is dependant on the speed of pouring
‘Although this is a very common and seemingly simple effect, it is remarkably difficult to explain it exactly within the framework of fluid mechanics,’ said paper author and fluid mechanics expert Bernhard Scheichl of TU Wien.
The mathematics that governs tea flow from the cup of a pot are complex and involve an interplay between capillary force and inertial forces, according to the team.
These forces are what hold the grains of sand together and permit tissues to take up water.
The former is responsible for ensuring that flowing fluid maintains its original direction.
This is countered by the latter which slows down the liquid at the ‘beak” at the end the teapot’s spray, creating a larger drop.
So, when the capillary forces are strong enough, the tea spills rather than pours — and this switch occurs for a given teapot at a specific contact angle between the spout and the flowing liquid surface.
The smaller the angle is, the team explained — or the more wettable (or ‘hydrophilic’) the material of the teapot — the more the detachment of the liquid flow from the spout is slowed down.
Dr Scheichl stated, “We now have a theoretical explanation for why this drops forms and why it always stays wetted underside,”
The teapot effect is influenced by gravity. Dr Scheichl, along with his coworkers, also examined the impact of gravity on the effects, and concluded that gravity does not play a significant role in them.
Model of flow from the inside edge of a Spout. It is important to identify the area where flow separates from the beak.
The fluid jet’s direction is determined by gravity, the researchers noted. However, the strength of the fluid is not important to its development.
Given this, they said, astronauts taking tea in a moon base would need to be careful about how they pour out their cups — as the teapot effect could still manifest in reduced gravity — but it would be a problem on a deep space station in zero gravity.
Journal of Fluid Mechanics published the full results of this study.