Bubbles wreak havoc in fluids and labs

Bubbles wreak havoc in fluids and labs

Finally a major mystery of everyday science that is clearing up. In a champagne flute or even when pouring water from a carafe into a glass, the rise of bubbles to the surface is often not in a straight line. The strings of bubbles sway, zigzag, even spiral as they rise. This dance had even intrigued in the XVe century Leonardo da Vinci, who had drawn it in his notebooks. Since then, convincing explanations for this strange effect have been lacking.

Just as there was a lack of clear experiences to highlight it. It was not until 1995 that Paul Duineveld, from the University of Twente (Netherlands), observed the swaying of the bubbles in water rid of all impurities, to which the zigzags of the bubbles were generally attributed. In addition, the experiment brought a surprise: small air balloons spin straight only if their diameter is less than 1.8 millimeters.

On March 10, in the Journal of the American Academy of Sciences, PNASa French team from the Institute of Fluid Mechanics in Toulouse explains not only this critical size, but also the scenario behind the tortuous rise of the bubbles.

End of the story ? Maybe not, because this article is a polite but firm answer to an article published by this same newspaper, on January 17, with an entirely different explanation. ” We we arrive at the same critical size. But let us differ on the main origin of the phenomenon »sums up Jacques Magnaudet, head of the Toulouse team, who has been studying bubble dancing for nearly twenty years. “We completely disagree with them regarding the script”responded separately to Monde the two authors of the January article, Jens Eggers (University of Bristol) and Miguel Herrada (University of Seville).

Different hypotheses

Three hypotheses are competing to destabilize the rectilinear rise of the bubbles. Firstly there is the one that allows a spinning ball to adopt a curved trajectory. We will speak of wake instability in reference to the effects of the air along the walls and behind the object.

Then there is that involving the deformation of the bubble whose wall inflates and deflates during the ascent. Finally there is that known as the fluid-object coupling, which considers that the bubble is an object free to move in a fluid: the fluid disturbs the object which itself disturbs the movement of the fluid.

In 2002, Jacques Magnaudet carried out numerical simulations which showed that the wake effect exists for bubbles. The appearance of vortices downstream creates instability and a force that deflects the rise of the bubble. Calculations predict that this only happens for bubbles above a certain threshold, about 20% above the observed 1.8 millimeter diameter. “For ten years I thought we had solved the problemremembers Jacques Magnaudet. But it was wrong. The critical radius was too big and above all the induced trajectory was not in zigzag but in “crab”. »

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