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Quantum in the flow of water

Simulation of a water flow (in red and pink) in the middle of a carbon nanotube (in gray).

The level rises. Soon to become a plumber, you will have to master quantum physics, this theory of particles which one wonders how it would help to solve pipe problems. However, without it, it is impossible to solve a fifteen-year-old plumbing problem, according to an article published in Nature, February 3.

For several years, physicists have discovered that water flows 10,000 times faster in microscopic tubes of a few tens of nanometers in diameter made of carbon. But this property disappears if, instead of carbon, a mixture of boron and nitrogen is used even though the surface looks equally smooth. In addition, unexpectedly, in the “large” tubes of more than thirty nanometers in diameter, the water rubs more than in the small ones and it is therefore slowed down.

To explain these phenomena, Lydéric and Marie-Laure Bocquet (ENS Paris) with Nikita Kavokine (ENS Paris and Institut Flatiron in New York) took advantage of periods of confinement to delve into scientific literature, including grimoires that their specialties , chemistry and hydrodynamics, were unaware of until then. And the ideas flowed.
First, they thought that the surface acts on water like corrugated iron. But, between carbon and the boron-nitrogen mixture, there is no geometric difference, whereas the flow is slower in the second case.

Diving into the sea of ​​electrons

They then tested a first quantum hypothesis. In the material, the electrons which “come out” a little from the surface and attract electrical charges from the water could slow down its flow. But this effect does not fully explain the phenomenon. They then dived deeper by looking at the material’s sea of ​​electrons known to be stirred up by light “waves”, called plasmons, which could “dialogue with the fluid”, as the authors of the study put it. This time everything is explained! The liquid rubs on the surface so the ripples within it resonate with the plasmons. “It’s like when you wiggle your feet in your bath. If you beat fair, it can really overflow! », dares Lydéric Bocquet. As the plasmons of carbon and boron-nitrogen do not have the same frequencies, the tub does not overflow in the first case and therefore little energy is lost: the fluid hardly rubs.

Ditto for the effect of the diameter of the pipe. Varying it is like tuning a guitar. At a large diameter, the waves in the water and those of the plasmons agree: the liquid rubs. At small diameter, there is disagreement, no energy dissipated, and the fluid slides without friction.
“This model explains non-trivial experimental facts. It combines elements of electronic theory of solids to explain a phenomenon of fluid mechanics, which usually corresponds to two communities speaking little to each other”appreciates Thierry Giamarchi, professor at the University of Geneva.

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