Extending work older by more than a century, researchers have characterized the flow regime which leads to the formation of a sort of chain made of liquid links.
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The phenomenon is most banal, observable by everyone on a daily basis, and yet he has just found a complete description in an article of Physical Review Fluids , signed on October 11 by a Dutch team from the University of Amsterdam. It is a question of understanding why the water net of a bottle does not flow according to a very smooth cylinder, but will bother at regular intervals, adopting the form of a succession of links of a liquid chain. The higher the flow, the more the links are spaced. And the greater 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) has shown that a liquid jet is unstable and that he ends up breaking into drops. Niels Bohr, Nobel in 1922 for his contributions to quantum mechanics (useless in this case), he, during his master in 1909, explained how Rayleigh’s instability creates liquid chains , if the flow nozzle is not symmetrical.
The reason comes from the surface tension, this force which connects the water molecules between them, to the point of managing to support pepper grains or water spiders. These forces seek to minimize the liquid surface in contact with the air. Thus, if the shape of the flow is asymmetrical at the start, like an oval (or ellipse), the forces tend to round it, until the oval and the dominant forces change their orientation. And so on, giving birth to a liquid chain.
“The hardest part was to make a flow without turbulence and laminar. See, on a meter high, about twenty links is very beautiful”, underlines Daniel Bonn, professor at the University of Amsterdam . The team has multiplied the forms thanks to a dozen more or less elliptical nozzles.
Conclusion: Niels Bohr was right. But the researchers went further than their elder considering several modes of action of the surface tension forces. They can indeed “pinch” in two points (as in the explanation of Bohr) or in three, four, five … Complete digital simulations have made it possible to precisely describe the forms of the jet, some details of which are only explained by the effect of several modes of action of the 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,” said Jens Eggers, from the University of Bristol .
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