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Cavitation in hydrogels biomimetic cellular networks and wetting of hybrid surfaces

Xavier Noblin Laboratoire de Physique de la Matière Condensée, CNRS-UNS, Nice.

I will present two topics where the physics of interfaces between liquid, vapor and solid play the main role through bubbles dynamics and droplets wetting.

In a first part I will describe an experiment on cavitation in microfabricated hydrogels 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 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.

In a second part, I will describe the fabrication and wetting properties of new kind of hybrid surfaces. Superhydrophobic surfaces have been developed following a biomimetic approach for two decades by adding physical roughness (eventually at various scales) to chemical hydrophobicity. We have demonstrated how this approach can be developed using electropolymerisation [3]. Here I will present how we did take advantage of this method to fabricate hybrid surfaces (superhydrophobic and hydrophobic/hydrophilic parts). Cylindrical pillars of 15 microns in height are surrounded by superhydrophobic domains. We varied two main parameters : the inter-pillar spacing and the amount of conductive polymer added. I will describe the wetting properties of these surfaces as function of the parameters varied and compare with a mix Wenzel/Casse-Baxter model.

 


Cavitation bubbles in a fern sporangiumand in a hydrogel device (left) and a millimetric drop on a hybrid surface (right)

 

[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).
[3] T. Darmanin et al. Superoleophobic behavior of fluorinated conductive polymer films combining electropolymerization and lithography. Soft Matter (2011).