Bubble Formation by Air Fingers in Multi-Plunging Jets

Authors: Narendra Dev, Helene Scolan, J John Soundar Jerome, and Jean-Philippe Matas
(Univ Lyon, Univ Claude Bernard Lyon 1, CNRS, Ecole Centrale de Lyon, INSA Lyon, LMFA, UMR5509, 69622 Villeurbanne France)

The video first shows multiple identical water jets, totalling 61, each 2.7 mm in diameter, impacting a water pool at 1.3 m/s. An injector plate generates these jets in a tightly packed hexagonal pattern. In the next part, the video reveals the scene below the water’s surface. An inverted dome forms just beneath the surface, surrounded by several slender, air-finger-like structures. These microscopic fingers undergo a repeated cycle of pinching o bubbles and retracting. Both footage of the jets and the ff dome are captured at 20,000 FPS.

Transportation of air into the liquid phase in the form of bubbles is a widely known phenomenon. This ubiquitous and complex process facilitates the transfer of oxygen and other gases when a breaking wave plunges into the ocean, essential for sustaining aquatic life. At the fundamental level, the air-entrainment mechanism can be examined by considering a simple case of a plunging jet impacting the surface of a pool. This impact generates a bubble-laden jet ow underneath the surface, known as a bubble cloud [1, 2]. As the video shows, we experimentally investigate hexagonally packed jets to mimic air entrainment in fragmented plunging jets, like waterfalls. Surprisingly, a 3D inverted dome structure forms just below the interface, unlike in the single-jet case. Phase detection optical probes reveal that the interior of this unique two-phase structure consists of liquid jets encased in an air jacket. This indicates that the water level inside the dome is noticeably lower than the surrounding, forming a depressed meniscus. High-speed footage shows that the lateral surface of the dome is surrounded with several microscopic-sized air ngers, formed by air trapped between the jets as they mix the bulk water. These ngers utter, pinch o bubbles, and retract with a whip-like motion, continuously repeating the process and generating countless bubbles. It is speculated that dome forms through the interaction of multiple air pockets sur- rounding each jet, a phenomenon that has never been observed, described, or quanti ed. Our research aims to determine whether the breakup mechanism of air nger is Rayleigh- Plateau (capillary-driven) or turbulence-driven. This insight is expected to improve the modelling of bubble size and the air-entrainment process.

References:
[1] Narendra Dev, J John Soundar Jerome, Helene Scolan, and Jean-Philippe Matas. Liquid inertia versus bubble cloud buoyancy in circular plunging jet experiments. Journal of Fluid Mechanics, 978:A23, 2024.
[2] G. Guyot, A. Cartellier, and J.-P. Matas. Penetration depth of a plunging jet: from microjets to cascades. Physical Review Letters, 124(19):194503, 2020.