The phenomenon is very commonplace, observable by everyone on a daily basis, and yet it has only just found a complete description in an article by Physical Review Fluidssign October 11 by a Dutch team from the University of Amsterdam. The aim is to understand why the trickle of water from a bottle does not flow in a very smooth cylinder, but bubbles up at regular intervals, taking the form of a succession of links in a liquid chain. The stronger the flow, the more the links are spaced. And the bigger the neck, the wider these links.
The explanation has been known for more than a century, thanks to eminent physicists. Lord Rayleigh (Nobel in 1904 for anything else) showed that a liquid jet is unstable and eventually breaks into drops. Niels Bohr, Nobel in 1922 for his contributions to quantum mechanics (useless in this case), explained to him, during his master’s in 1909, how Rayleigh instability creates liquid chains, if the flow nozzle does not is not symmetrical.
The reason comes from the surface tension, this force which binds the water molecules together, to the point of supporting peppercorns or water spiders. These forces seek to minimize the liquid surface in contact with the air. Thus, if the shape of the flow is initially asymmetrical, like an oval (or ellipse), the forces tend to round it, until the oval and the dominant forces change orientation. And so on, giving rise to a liquid chain.
Research aimed at improving the manufacture of sprays or inkjet printers, for example
“The hardest part was achieving a turbulence-free and laminar flow. To see, over a meter high, about twenty links, it’s very beautiful », underlines Daniel Bonn, professor at the University of Amsterdam. The team multiplied the shapes using a dozen more or less elliptical nozzles.
Conclusion: Niels Bohr was right. But the researchers went further than their predecessor by considering several modes of action of surface tension forces. They can indeed ” pinch “ in two points (as in Bohr’s explanation) or in three, four, five… Complete numerical simulations have made it possible to precisely describe the shapes of the jet, certain details of which can only be explained by the effect of several modes of action of surface forces. “Even if the problem is classic, there has not been a lot of work on it, because controlling and measuring these flows is difficult. This work shows significant progress in this area.”says Jens Eggers of the University of Bristol.
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