Easy to observe, difficult to explain: If we are too hesitant about pouring a cup of tea, the liquid will run down the spout of the pot – it will spill. Scientists have now discovered for the first time how this so-called “teapot effect” is composed. A small drop on the underside of the beak is therefore crucial. This can divert the entire tea flow.
Who does not know it: When trying to pour tea into a cup, the drink often runs down the neck of the pot and drips onto the tablecloth. To prevent this, it helps to pour the tea with more energy. What sounds trivial at first has posed major problems for scientists for several decades. The engineer Markus Reiner described the “Teapot effect” for the first time as early as 1956, but could not definitively explain it physically.
Small drop, big effect
“Although it is a very everyday and apparently simple effect, it is remarkably difficult to explain it exactly in terms of fluid mechanics,” says Bernhard Scheichl from the TU Vienna. A team led by Schleichl has now completely described the phenomenon mathematically in a study for the first time. Through complex fluid mechanical analyzes and experiments, they were ultimately able to find the culprit.
According to the results, a small drop on the underside of the teapot’s beak is responsible for the mess. It inhibits the flow of tea and directs it along the wall of the pot instead of into the cup. “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 Schleichl.
Speed and spout are decisive
According to the study, the size of the disruptive drop depends crucially on the so-called Weber number. It describes the relationship between the inertia of the liquid and the surface force and is dependent on the square of the speed of the tea. If the tea is poured too slowly from the pot, the surface tension will prevail and the jet will be deflected along the edge of the beak. Then the tea drips onto the table.
According to Schleichl and his colleagues, the flow speed of the tea does not only depend on the inclination of the pot. Capillary forces on the porcelain surface decelerate the liquid decisively. In addition, the smaller the angle at the break-off edge of the beak, the more likely the flow of liquid will stick to the surface.
Even on the moon
Interestingly, gravity plays only a minor role in this effect. It just determines the direction in which the drop forms on the edge of the beak. How strong the gravity is doesn’t make much difference. So if we were to pour ourselves a cup of tea on the moon, the teapot effect would be observed in a similar way.
However, if there is no gravity – as is the case on a space station, for example – no drop forms and we can safely pour ourselves a cup. (Journal of Fluid Mechanics, 2021; doi: 10.1017 / jfm.2021.612)
Source: Vienna University of Technology