2023-11-29 16:00:07
Water flowing from a tap forms a circular “wall” a few millimeters high. GOERLINGER ET AL.
A team from the Institute of Electronics, Microelectronics and Nanotechnology in Lille has discovered a phenomenon that had never been spotted before. Neither by the attentive eye of Leonardo da Vinci, in the 16th century, nor by that of a master of fluid mechanics, Lord Rayleigh, in 1914, both of whom had nevertheless looked into the subject.
The latter, however, is quite commonplace and everyone has already seen it in their sink or washbasin: the hydraulic jump. Or how, around the jet falling from the tap, a liquid bead forms at the bottom of the container surrounding a smooth and thin area. The “jump” designates this liquid ring, which Leonardo da Vinci had perfectly drawn. The novelty is that this ring, which persists as long as the water flows, can, under certain conditions, disappear, then reappear, then disappear again. Specialists say that the jump oscillates.
“We may not be the first to have seen it, but no one had published and explained it before us, in any case, notes Alexis Duchesne, teacher-researcher at the University of Lille and co-author of the publication published in Physical Review Letters, November 8th. “It’s great to find something new in such an old phenomenon”appreciates co-author Michael Baudoin, professor at the University of Lille.
Closing a controversy
Another pleasure, the discovery is, so to speak, fortuitous. The researchers did not set out to see the famous liquid bulge disappear. On the contrary, they wanted to close a controversy over the very origin of the jump. Since 2018, in fact, and a theoretical proposal by Rajesh Kumar Bhagat of the University of Cambridge, debates agitate the community on the reasons for the appearance of a wall of water. Is this the equivalent of breaking the sound barrier, with a liquid propagating faster than the waves on its surface, as we thought before 2018? Is it a sudden “dewetting” of the liquid under the effect of surface tension forces, known to give its shape to the meniscus of water in a container, for example, as the Briton suggests?
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“We launched small-scale experiments, with jets less than a millimeter in diameter, precisely to better understand the role of surface tensions, tell Alexis Duchesne. But, to understand the oscillation phenomenon, we do not need to know the origin of the jump. »
So they didn’t answer this question, but came across something else: the oscillation of the ring. It’s a story of waves. When the jump is “installed”, it looks like a cavity with the trickle of water falling, then a smooth internal area, then a “wall of water”, and finally a thicker liquid zone. The ripples which propagate in this cavity can therefore be amplified by successive back and forths. These waves move the liquid, creating dips and bumps. If the flow is low enough, these variations in altitude can cause the approximately 4 millimeters of height of the wall of water to disappear. And make them reappear less than a quarter of a second later.
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