Have you ever noticed that Brazil nuts often end up on top of your nut mixture?
However, they are the heaviest… why don’t they clump together at the bottom of the pot? A European team has studied this counterintuitive phenomenon, dubbed the “Brazil nut effect.”
If curious, it is not only about nuts: the effect is also verified when a mixture of grains of different sizes is subjected to vibrations. The larger, and often the heavier ones, tend to rise to the surface, as if floating on top of the others. the reason ? When such a mixture is agitated, spaces are created between the grains so that the smaller ones can force their way to the bottom of the container, thus pushing the larger grains upward.
However, for the first time, scientists from the Universities of Utrecht and Warsaw have observed that this effect occurs without tremors, and therefore without external energy input.
The experiment was carried out on a colloidal mixture, that is, very fine particles suspended in a liquid. In this case, the mixture consists of oil and particles of about 1.3 and 2.8 microns in diameter. Brazil nuts played the role of the largest.
“A micron is 100 times smaller than the diameter of a human hair,” recalls David Vidal, a professor in the department of mechanical engineering at Montreal Polytechnic who was not involved in the study. These particles are so small that they are affected by molecular agitation and electrostatic forces. »
In fact, the molecules of the oil vibrate when they are in the liquid state, and the molecules of the mixture are so small that this agitation is sufficient to move them as well. Electrostatic forces also contribute to this motion: their surface is positively charged, and the particles repel each other. The larger ones, carrying a higher load, repel more than the smaller loads, which pulls them more easily to the surface.
Thus, even without an external power supply to the system, forces occurring at this scale may be sufficient to separate the particles according to their size.
this discovery, Posted in PNASThey can especially help extend the life of paints. “When you leave it on for too long, the paint peels off,” says Professor Vidal. but [en tenant compte du phénomène], one can succeed in creating a mixture with a structure that remains stable. »