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Impact and rebound of liquid drops: from fundamental to technological aspects

Final Activity Report Summary - IMPACTING DROPS (Impact and rebound of liquid drops: from fundamental to technological aspects)

The project has yielded various results in the field of free surface flows, with direct impact for technological applications. First, we have explained a long-standing controversy on the dynamics of bubble pinch-off. This fundamental problem in singularities focuses on the final instances when an elongated bubble breaks up into two smaller bubbles. This breakup also happens for drop impact on a hydrophobic substrate, where it leads to the formation of violent jets. The fluid mechanical equations become singular at this point and existing experimental and numerical results disagreed on the precise dynamics. We developed a theory that shows that there is a universal dynamics, but that this regime is approached very slowly, explaining the apparent non-universal behaviour in the literature.

Second, we found a new solution to the classical problem of dip-coating, a common technique to cover a thin film of liquid onto a hydrophobic surface. It has been thought since the work by Landau and Levich in the 1940s that the thickness of the coated film is uniquely determined by the speed of withdrawal of the solid. We found a new set of mathematical solutions that allow for a range of thicknesses. Our colleagues from the ESPCI in Paris have confirmed that these new films are easily generated experimentally, providing a way to obtain coatings that are much thicker than the Landau-Levich films. We furthermore showed how this coating is closely related to the stability of the moving contact line.

Third, we studied drops that are levitated by an air cushion. The most famous realization of this phenomenon is encountered in the Leidenfrost-effect for which water drops can 'float' above a hot plate. This principle is also used in the production of lenses, where molten glass is cooled by an air cushion. We identified the mechanism of instability that is observed in these systems, which is particularly hindering the lens production. By computing for the first time the detailed shape of the drops, we found the critical volume at which drops become unstable.