Xavier Noblin (INPHYNI, Institut de Physique de Nice). Cavitation avalanches in natural and synthetic 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, or acoustic conditions. Quasi-static conditions can also be used, they 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]. I will first introduce this topic during the talk over several examples. I will emphasize the issues for trees of these phenomena on their resistance to drought events.

Both natural systems (trees and fern sporangia) 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 a very fast collective nucleation of bubbles in all the cells. We study this mechanism in hydrogels-based biomimetic devices. They are made of 2D networks of water-filled cavities using soft lithography and pHEMA-MMA 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 studied this phenomenon using high speed imaging and acoustical techniques. We have also developed theoretical model and numerical simulations.

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.

[1] X. Noblin et al., The fern sporangium: a unique catapult, Science (2012).
[2] T. D. Wheeler and A. D. Stroock, The transpiration of water at negative pressures in a synthetic tree, Nature (2008).