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What happens when we pour a liquid into a
container? Work done by researchers at the "Laboratoire des Fluides
Organisés" (CNRS – Collège de France) will help
us to answer this question; they have actually identified the parameters
that are involved in the formation and the fracture of the tip that appears
when this action occurs.
In their work, the researchers analyzed
what happens when a liquid hits a bath of liquid of the same nature, exactly
like what happens when we fill a container or a mold or when the bow wave
of a ship crashes. At low impact, the shock slightly hollows the region.
Its bottom becomes extremely sharp when the impact velocity is increased.
To the naked eye, this tip appears to be extremely sharp, despite the
regulating action of the surface tension that prevents these types of
regions from occurring. By observing this tip more closely with a microscope,
we can actually measure its radius of curvature whose order of magnitude
is 10 microns. Scientists have also shown that this curvature decreases
exponentially with the impact velocity: a slight increase in the impact
velocity of the liquid leads to a considerable decrease of the radius
of curvature. This law, which predicts the appearance of a characteristic
tip for a given velocity, is in agreement with the theory developed by
the physicist Moffatt(1) ten years ago but which has never been experimentally
validated.
An exponential decay curve law would imply that by slightly increasing
the impact velocity, we can generate a molecular tip (that is, the smallest
tip imaginable). However, researchers have shown that this is not the
case: if the velocity is too great, the point gives way and a film of
air is entrained inside the pool. This air is responsible for the formation
of foam in the case of the bow wave and leads to the formation of bubbles
when a diver hits the water in a swimming pool. Scientists have characterized
the detection threshold of the film of air and shown that it does not
depend only on the surface tension and viscosity of the pool, but also
on the viscosity of the upper phase (air as well as oil in their experiments).
An argument put forth by the physicist Eggers(2) makes it possible to
understand this phenomenon: if the tip gives way, it is because it is
subject to the lubrification pressure of the upper fluid, forced to be
entrained and then released from the region which is more and more confined
as the velocity increases.
The consequences of these results are obvious for industry: generally
speaking, when we fill a mold or a container, we try to do so as quickly
as possible, while also avoiding the formation of bubbles. It is therefore
very important to understand what makes the bubbles appear.
Reference:
E. Lorenceau, F. Restagno and D. Quéré. Fracture of a viscous
liquid. Phys. Rev. Lett., 90 (18) :184501 (2003)
1 - English physicist and mathematician
specialized in fluids and vortexes.
2 - German physicist who did a great deal of work on phenomena that occur
when drops of water are formed.
Researcher contact:
David Quéré
Tel: +33 1 44 27 10 79
E-mail: david.quere@college-de-france.fr
Press contact:
Muriel Ilous
Tel: +33 1 44 96 43 09
E-mail: muriel.ilous@cnrs-dir.fr
Mathematics and Physical Sciences Department
contact:
Frédérique Laubenheimer
Tel: +33 1 44 96 46 23
E-mail: frédérique.laubenheimer@cnrs-dir.fr
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