It must have happened to you more than once: very slowly and carefully you try to pour the drink from the kettle into the glass, but instead of a deliberate hit, the jet slides on the underside of the spout and everything spills. It seems an annoying omission. But no – this is a regularity! The effect of the kettle has been studied for decades and now a research team has been able to describe it in detail after careful theoretical analysis and numerous experiments.
Scientists have found that the interaction of different forces keeps a small amount of liquid directly on top of the spout: this is enough to redirect the flow of liquid under certain conditions, writes National Geographic. The effect of the kettle was first described by Marcus Rainer in 1956, and scientists have repeatedly tried to explain this effect. Work on this topic was even awarded the humorous Schnobel (Anti-Nobel) Prize in 1999.
The study of the effect of the kettle has now been finalized at Rainer’s Alma Mater, Vienna University of Technology. “Although this is a very common and – it seems – simple effect, it is extremely difficult to explain from the point of view of fluid mechanics. The edge on the underside of the spout of the kettle plays an important role: a drop forms there, leaving the area just below the edge always wet. The size of this drop depends on the speed at which the liquid flows out of the kettle. If the velocity is below the critical threshold, the flow is redirected and the liquid flows down the spout, “said Bernhard Scheichl, lead author of the study. “For the first time, we were able to give a full theoretical explanation of why this drop forms and why the underside of the top of the spout always stays wet. The mathematics underlying this is complex – it is the interaction of inertial, viscous and capillary forces. The force of inertia ensures that the liquid tends to maintain its original direction, while the capillary forces slow down the movement of the liquid right next to the spout of the kettle.
The interaction of these forces underlies the so-called kettle effect. But the capillary forces ensure that the effect begins only at a certain angle between the wall and the surface of the liquid. The smaller this angle or the more hydrophilic the material of the kettle, the slower the separation of the liquid from the surface of the spout, “adds Sheikhl. Interestingly, the force of gravity relative to other emerging forces does not play a decisive role. Gravity simply determines the direction in which the jet is directed, but its strength is not crucial to the effect of the teapot, so the effect of the teapot will be observed even when drinking tea on a lunar basis (but not on the International Space station where there is no gravity at all).
