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Finally, scientists explain the ‘teapot effect’, the dribbling of the liquid outside the teapot

Scientists have now explained the 'Teapot effect' which takes place when pouring liquid from the teapot too slowly, it at times dribbles down outside the teapot, making the spotless white tablecloth dirty (Pic. Courtesy gigazine.net)

There are several situations and phenomena which are intriguing but can’t be explained, at least for the time being. One such was the “teapot effect”.

“Teapot effect” takes place when pouring liquid from the teapot too slowly, the liquid flow at times does not disengage from the teapot, making it into the cup but with the liquid dribbling down outside the teapot. This since time immemorial has made spotless white tablecloths dirty.

While several scientific studies over many years were done to explain the effect, it is now a team of researchers from TU Wien has been able to do successfully, according to a report in scitechdaily.com. The team has comprehensively described the “teapot effect” with not just theoretical examination but also several experiments. According to them, the interaction between varied forces keeps a small quantity of liquid at the edge directly, and this is enough for the flow of liquid to be redirected under certain conditions.

History of “teapot effect”

Markus Reiner in 1956 described the “teapot effect”. In 1913 he got his doctorate from TU Wien, following which he moved to the US. There he emerged as an important pioneer in the science of flow behaviour or rheology. Several scientists made numerous attempts to explain this effect precisely and work on this issue received the satirical “IG Nobel Prize” in 1999.

With the team of TU Wien now doing the explanation, the teapot effect has come full circle. The team which did it included Dr. Bernhard Scheichl, who is a Key Scientist at the Austrian Centre of Excellence for Tribology (AC2T research GmbH) and a lecturer at the Institute of Fluid Mechanics and Heat Transfer and it was done in collaboration with University College London’s Department of Mathematics.

Talking about the study, Scheichl said: “Although this is a very common and seemingly simple effect, it is remarkably difficult to explain it exactly within the framework of fluid mechanics.”

On the underside of the teapot beak there is a sharp edge and that plays a significant role. A drop forms, thus keeping the area directly below the edge permanently wet. The drop size is dependent on the speed of the flow of the liquid out of the teapot. Lower speed of the liquid than the critical threshold will result in the entire flow around the edge and dribbles on the teapot’s outside wall. “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,” said Scheichl.

As for the mathematics involved behind this, it is rather complicated. It is an interplay of viscous, capillary forces and inertia. It is the inertial force that makes sure that the fluid tends to maintain its original direction, while right at the beak the capillary forces slow the fluid down. Interaction of these forces is what causes the teapot effect. The effect commences only at a specific contact angle between the wall and the liquid surface and this is ensured by the capillary forces. When this angle is smaller or the more wettable the material of the teapot is, the more the detachment of the liquid from the teapot is slowed down.

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What is significant is that gravity’s strength in relation to other forces that occur, doesn’t have a decisive role to play. While gravity decides the direction in which the jet is directed, its strength does not play a decisive role in the teapot effect. So this effect will take place even when one is drinking tea on a moon base but will not happen in a space station, the reason being absence of gravity there.

The teapot effect’s theoretical calculations were published in the Journal of Fluid Mechanics. Experiments too were conducted, in which water from an inclined teapot was poured at different flow rates and this flow was documented on a film through high speed cameras. Through this it became possible to exhibit exactly how the wetting of the edge below a critical pouring rate results in the “teapot effect”.