Xavier Noblin - Institut de Physique de Nice, UCA, CNRS UMR7010 - Keywords: Fluid mechanics, Bubbles, Plant Biomechanics

Xavier Noblin - Institut de Physique de Nice, UCA, CNRS UMR7010 - Keywords: Fluid mechanics, Bubbles, Plant Biomechanics

Contribution title: Cavitation avalanche in natural and artificial devices

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]. 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 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 present experimental results on natural and artificial devices from ultra high seed imaging and acoustics measurements. We have also developed theoretical models and numerical simulations of this complex avalanche phenomenon mixing several spatial and time scales along with the number of nucleation events and the presence of three phases in the system (vapor, liquid, solid walls). The complexity arises from the interplay between the stochasticity of bubble nucleation, their nonlinear oscillations dynamics, their acoustical emissions and the mechanical coupling with solid walls.
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)."