Researchers at a university in Vienna used complex maths to figure out three different things that can lead to the ‘teapot effect’ – which is when the pouring causes spillage
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Scientists have finally unlocked the secret of pouring the perfect cuppa – without dripping from the tea pot.
The pourer must get the angle of the spout just right to avoid spillage, according to a study.
The reasons for the ‘teapot effect’, first described by Markus Reiner in 1956, have proven a challenge to scientists for decades.
It is down to complex fluid dynamics which make pouring liquids from teapots a delicate process.
If poured too slowly, the flow can dribble down the outside of the teapot rather than landing in the mug.
Experts at the Institute of Fluid Mechanics at Vienna University of Technology discovered why with a study.
They then did some further experiments to check out their findings.
Study author Dr Bernhard 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.”
Water was poured from a tilted teapot at different flow rates and filmed with high speed cameras to assess the perfect non-drip method.
Experts in rheology, the study of flowing matter, used ‘complicated’ maths formulas to calculate three different forces – inertia, viscous and capillary, which cause the teapot effect.
While inertia force ensures the poured liquid maintains its original direction, capillary forces slow the fluid down right at the teapot’s beak.
A drop formed on the underside of the teapot beak, the size of which depended on how quickly the liquid was flowing.
If the speed was below the ‘critical threshold’, the drop redirected the entire flow around the edges and down the sides of the teapot.
Dr Scheichl added: “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.
“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.”
Only when the pot’s wall is at a very specific angle in relation to the liquid’s surface does the mysterious effect start, the research found.
The smaller the angle, the harder it becomes for the liquid to detach itself from the spout. Dr Scheichl said: “The teapot effect would be observed on a moon base, but not on a space station with no gravity.
“It was possible to show exactly how the wetting of the edge below a critical pouring rate leads to the ‘teapot effect’, confirming the theory.”
The study recommended pouring tea without delay when freshly brewed, at 80C, because the liquid is less likely to adhere to the vessel. Tea pots with a long, slender spout, made from china, ceramic or glass, were best to avoid spills.
Tea is by far the most popular drink, as Britons enjoy around 100m cups every day.
Dr Scheichl worked with the maths dept at University College, London, and findings were published in the Journal of Fluid Mechanics.