RESEARCH

In the context of a globalization every day more international, a brand new paradigm shift was needed to tackle innovative issues, with the most modern of Marketing and Management technichs: Morphodynamics. In the best tradition of post-modernism, Morphodynamics is a new name for the study of instabilities and non-linear dynamics in a wide variety of natural and lab context: wetting, geophysics, biology, granular matter.

Contemporary problems in macroscopic Physics are based on the complexity of the systems under study. This complexity can arise from to the large range of scales involved, like in the problems of turbulence and of moving contact lines that I have investigated. It can result from the coexistence of a large number of dynamical
mechanisms, like in the natural systems that we have studied in botanics (the characterisation of leaf venation) and in geophysics (dunes morphogenesis and avalanche dynamics). Lastly, in out-of-equilibrium systems likegranular media, which are athermal model systems, the complexity is due to the failure of Statistical Physics and to the lack of a generic framework of description.

The field of Non-Linear Physics has undergone a renewal in the last few years, shifting towards problems out of its traditional territory. My research activities can be placed within this context. We have developed an activity of fundamental research at the interface with Earth Sciences : we have studied the dynamics of dunes
and avalanches, trying to extract some purely physical problems from these geolological objects. With such a cooperative vision, we have succeeded to create strong links with some members of the Geophysical community, ensuring an effective reception of our work. We have passed more than six months to study the dunes in the AtlanticSahara (Morocco) and, more recently, in the Grand Erg Oriental (Algeria). During these field missions, we have tracked the evolution of dunes using physical marks, GPS measurements and aerial photography (conception of a photographing kite and historic aerial views), measurements using home made sensors of the wind velocity, the sand flux, the erosion rate, the 3D topography, real field experiments (study the reaction of dunes to controlled disturbances, field wind tunnel, etc). We have recently started a collaboration with BBC TV to install slow video cameras in the desert to film the propagation and the interaction of dunes. When we started to work on dunes, the dominant vision in the community was essentially based on static features. We have identified the dynamical mechanisms governing the physics at the scale of the grain (modelling of sediment transport in a turbulent boundary layer) and at the scale of dunes (dynamical mechanisms, characteristic time and length scales of the problem). We have then showed how this could be used to design a properly rescaled experiment in the lab. Finally, we have studied the non-linear interaction between dunes and showed that the selection in size at the scale of the dune field results from dynamical effects.

A second important characteristic of my work is that problems have been resolved by simultaneously using experimental methods, in the lab and in the field, theoretical models and numerical simulations. This use of transversal methods originates from the desire to submit the technics to the construction of scientific questions. Our recent study on the dynamical wetting transition ¡V the entrainment of a liquid film by a solid in motion that is not wetted by the liquid at thermal equilibrium ¡V is a good illustration of this principle.We have demonstrated that the macroscopic approach that was prevailing does not allow to correctly describe this transition, which in fact is governed by the formation of structures at mesoscopic scales (hydrodynamical quasi-singularity at the back of drops, formation of a capillary ridge, etc). On the basis of theoretical calculations, analytical or numerical, and of controlled experiments, we have finely characterised the transition and showed that it can be described by viscous hydrodynamics, applied from the molecular scale to the macroscopic scale.

Finally, if the objects that we have studied (grains, drops, leaves, rivers, cracks, aeolian ripples, dunes, singing avalanches) may appear disparate at first sight, my work is crossed by coherent lines on the questions (How do the patterns form in leaves venation, tidal river networks, cracks, etc ? Is there a close analogy between an avalanche front and a mobile contact line ? What selects the final size of dunes, of aeolian ripples, of avalanche fingers ?) as well as on the theoretical and experimental approaches (for instance, the linear stability analysis successfully applied to the digitation of avalanche fronts, the formation of aeolian ripples, the nucleation of dunes, the relaxation of mobile contact lines ; or the derivation of low dimensional models by integration over the depth, used to describe avalanches, liquid films, rivers, dunes, etc).

The invitations to give lectures in conferences organised in other communities than non-linear physics ¡V in Geology, in physical geography but also in acoustics and in microfluidics ¡V give some evidence for the success of this research method.