Why teapots always drip
Scientists answer an age-old question

Date:
November 9, 2021
Source:
Vienna University of Technology
Summary:
Why do teapots always drip? This phenomenon has been studied
scientifically for decades - now a research team has succeeded in
describing the 'teapot effect' completely and in detail with an
elaborate theoretical analysis and numerous experiments.



FULL STORY ==========================================================================
The "teapot effect" has been threatening spotless white tablecloths for
ages: if a liquid is poured out of a teapot too slowly, then the flow
of liquid sometimes does not detach itself from the teapot, finding its
way into the cup, but dribbles down at the outside of the teapot.


==========================================================================
This phenomenon has been studied scientifically for decades -- now
a research team at TU Wien has succeeded in describing the "teapot
effect" completely and in detail with an elaborate theoretical analysis
and numerous experiments: An interplay of different forces keeps a tiny
amount of liquid directly at the edge, and this is sufficient to redirect
the flow of liquid under certain conditions.

An effect with a long history The "teapot effect" was first described by
Markus Reiner in 1956. Reiner earned his doctorate at TU Wien in 1913
and then emigrated to the USA, where he became an important pioneer of
rheology -- the science of flow behaviour. Again and again, scientists
have tried to explain this effect precisely. Work on this topic was
awarded the satirical "IG Nobel Prize" in 1999. Now, research on the
teapot effect has come full circle, as it was studied at Reiner's alma
mater, the TU Wien, by a team around Dr. Bernhard Scheichl, lecturer at
the Institute of Fluid Mechanics and Heat Transfer and Key Scientist at
the Austrian Centre of Excellence for Tribology (AC2T research GmbH),
in cooperation with the Department of Mathematics at the University
College London.

"Although this is a very common and seemingly simple effect, it is
remarkably difficult to explain it exactly within the framework of fluid mechanics," says Bernhard Scheichl. The sharp edge on the underside of
the teapot beak plays the most important role: a drop forms, the area
directly below the edge always remains wet. The size of this drop depends
on the speed at which the liquid flows out of the teapot. If the speed
is lower than a critical threshold, this drop can direct the entire flow
around the edge and dribbles down on the outside wall of the teapot.

"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," says Bernhard Scheichl. The mathematics
behind it is complicated -- it is an interplay of inertia, viscous and capillary forces. The inertial force ensures that the fluid tends to
maintain its original direction, while the capillary forces slow the
fluid down right at the beak. The interaction of these forces is the
basis of the teapot effect. However, the capillary forces ensure that the effect only starts at a very specific contact angle between the wall and
the liquid surface. The smaller this angle is or the more hydrophilic
(i.e. wettable) the material of the teapot is, the more the detachment
of the liquid from the teapot is slowed down.

Tea in space Interestingly, the strength of gravity in relation to the
other forces that occur does not play a decisive role. Gravity merely determines the direction in which the jet is directed, but its strength
is not decisive for the teapot effect. The teapot effect would therefore
also be observed when drinking tea on a moon base, but not on a space
station with no gravity at all.

The theoretical calculations on the teapot effect were published by the research team in September 2021 in the Journal of Fluid Mechanics. Now experiments were also carried out: Water was poured from an inclined
teapot at different flow rates and filmed with high speed cameras. In this
way, it was possible to show exactly how the wetting of the edge below
a critical pouring rate leads to the "teapot effect," thus confirming
the theory.

Video of the 'teapot effect':
https://www.youtube.com/watch?v=jzZ2_Yh8c68&t=3s ========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. B. Scheichl, R.I. Bowles, G. Pasias. Developed liquid film passing a
smoothed and wedge-shaped trailing edge: small-scale analysis and
the `teapot effect' at large Reynolds numbers. Journal of Fluid
Mechanics, 2021; 926 DOI: 10.1017/jfm.2021.612 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211109120309.htm

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