• The fern sporangium catapult. 

With J. Dumais (OEB, Harvard University), J. Westbrook (Univ. Florida), M. Argentina, C. Llorens and N.O. Rojas (LJAD, Nice).
   
    A large group of ferns eject their spores using a catapult mechanism which make their dispersal very efficient. We have analyzed the mechanics of this natural catapult. High speed imaging have shown that the fast recoil motion is realized in two time scales, which allows a stop for a short time of the arm, ejecting efficiently the spores.

Under the fern leaves sporangia are packed together. These capsules are full of spores and surrounded by a row of cells: the annulus. This acts as an elastic beam which opens under dehydration of the cells contents. The large deformation induced is coupled to large negative pressures in the cells, leading at some point to the nucleation of cavitation bubbles. At this moment, the elastic energy is released, the annulus closes back very quickly, ejecting the spores. Ancient ages catapult present a crossbar that would stop the beam motion midway. Without it, projectiles would have been thrown directly into the ground.   We discovered that because of the porous nature of the annulus, the fast closing motion takes place in two time scale. The first one is inertial: it lasts a few tens of microsecond. The second one is poroelastic and lasts a few tens of milliseconds. This allows to eject the spores at more than 10 m/s.

In this movie (HQ: 768x576, 26Mo) (LQ: 384x288, 7Mo) we explain the fern sporangium ejection mechanism (Edited by Xavier Noblin, and © Science/AAAS).

• Ballistospores discharge. 

With S. Yang, J. Dumais.

We have studied the ejection of fungal spores from the basidiomycetes family. This ejection of spores is induced by fast capillary action due to droplet coalescence. The Journal of Experimental Biology's Outstanding paper prize 2009

In this movie (2Mo) we explain the ballistospores ejection mechanism.
See highlights in:

• The Journal of Experimental Biology's Outstanding paper prize 2009
• Eurekalert
• ScienceNews

• Vein density in artificial and real leaves. 

With M. Zwieniecki, N.M. Holbrook, L. Mahadevan, D.A. Weitz, I. Coomaraswamy.    

We have studied simple microfluidic devices mimicking leaves to better understand the geometric relationships between vein spacing and leaf thickness and their effect on transpiration. We have then measured the same paramters in real leaves, and observed the same scalings, showing the strong correlation between the thickness of a leaf and the minimal spacing bteween its veins.