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Cavitation propagation in natural and artificial (microfabricated) systems
The nucleation and growth of vapor bubble in a stretched liquid medium is a common phenomenon along boat helices. Main studies on cavitation in water under tension concern then hydraulics, but also acoustic or quasi-static conditions. These later are observed naturally in the sap conducting network of trees (xylem) or ferns sporangia where negative pressures lower than -100 bar are used in this catapult-like elastic beam [1]. It has also been observed in synthetic trees [2]. All these systems are compartmented and the way cavitation nucleation interacts between neighboring cells or cavities remain poorly understood. We observed that in the ejection of fern spores, the catapult mechanism is triggered by an almost instantaneous nucleation of bubbles in all the cells. We show that these events are correlated and propagate between nearby cell as a fast, collective avalanche phenomenon. We also study this mechanism in hydrogels-based biomimetic devices to better understand it. They are made of 2D networks of water-filled cavities using soft lithography and pHEMA hydrogels. We found that the nucleation of one bubble, that comes out randomly, can trigger subsequently the nucleation of several (up to hundreds) bubbles. We have also developed theoretical model and numerical simulation. Our results explain why the fern sporangium catapult can be so efficient since all the cells can cavitate in a few microseconds, it can also give insights in the way cavitation propagate in the microfluidic sap-networks in trees.