A record that does not lack air: researchers from the University of Lille have managed to make a spherical structure that held more than one year.
It is very rare to contemplate a soap bubble more than a few minutes, before it disgusts. So, having kept one in front of them for more than a year deserves some explanations. “She has even changed color, probably because of the development of a bacterial flora on the surface!”, I have fun Michael Baudoin, Professor at the Institute of Electronics, Microelectronics and Nanotechnology at the University of Lille, and co-author of this lifetime record published on January 18th in Physical Review Fluids.
In general, a bubble bursts for three reasons. The first is the easiest to control: the drilling by a tip or even a dust. The following two are more subtle. The wall of an air bubble is formed of a severity liquid and which will “flow” along the surface down, slimming the thickness of the top of the sphere. This fragility will eventually be fatal. The other phenomenon is the decrease in this thickness by evaporation of the wall of the wall – a way of prolonging the life of a bubble is also to spray a little water in the ambient air to limit the losses of the envelope water.
The Lille Recordmen have therefore managed to counter these two effects to make air bubbles of eight millimeters in diameter, one held 465 days. The first ingredient of the success had already been proposed in 2017 by a team from Paris-East University . It is a question of adding the tiny plastic particles (less than 100 microns in diameter) “wetting” but not too much, so that placed on the water they stay on the surface, without sinking. “This creates a crosslinked wall: the plastic grains are kept between them by liquid bridges, as in a water tower. The result is that the water flows less and that drainage is reduced,” summarizes Alexis Duchesne , teacher-researcher at the University of Lille, another co-author of the recipe. But these bubbles, nicely qualified as “marbles” in 2017, are ephemeral, even if they already hold several tens of minutes.
The researchers have thus thought of further reduction drainage by adding a second ingredient, glycerol, a viscous liquid, with a sweet taste, which probably explains the development of bacteria. And it walked … but not for the expected reason! Indeed, the team realized that the main cause of the bursting of plastic particle-enriched bubbles is evaporation and not drainage. And that glycerol, if it is in quite large quantity, reduces this phenomenon because it is hygroscopic, in other words, it likes water and absorbs that of the atmosphere around the wall of the bubble. It thus opposes natural evaporation until the structure finds its almost eternal balance.
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