Complex rheology fluids as analog of geological processes
Vendredi 14 février 2014
à 10 en salle C. Brot
A terrestrial planet is a complex system whose rheological layering is a consequence of variation of several parameters, such as composition, temperature, pressure and deformation. Because of the impossibility to look at the diverse physical phenomena buried within it and to follow their long-term evolution, indirect measurements are forcedly required, i.e. geochemical signatures, seismological images, geodetic and geological observations. However, these data provide only a static snapshot of the process. Therefore, use of laboratory modeling, employing real materials, is required to obtain dynamic information and to offer robust constraints to better understand the physics beyond geological processes.
In a laboratory model, according to ’similarity criteria’, both rheological and physical parameters of working materials must be properly scaled to natural conditions. It is, hence, pivotal to look for fluids that are able to reproduce in the experiments the whole spectrum of rheological behaviors, from brittle to viscoelastic to viscous. Thanks to the improvement of accurate and innovative instruments, laboratory experiments using well-controlled complex fluids are now achievable.
During my experiences I had the possibility to characterize different kinds of materials, from biopolymers, colloidal suspensions, yield stress fluids. The emergence of typical mechanical and rheological properties makes the tested materials suitable for modeling determined geophysical phenomena.
We found that gelatin has the required rheological properties to simulate crustal deformations, in particular the seismic variability of subduction thrust faults. On the other hand, by drying colloidal suspensions it is possible to obtain, for the first time in a laboratory, spontaneous asymmetric subduction. Our experiments evolve with time, showing a sequence of convective regimes that remind the different regimes observed in terrestrial planets. At last, thermal instabilities arising in yield stress fluid, i.e. Carbopol, show peculiar morphology and behavior that differ from mushroom-shape typically encountered in newtonian or purely shear-thinning fluids. Complementary numerical simulations are carried out. This type of hydrogels is a good candidate to get new insights into lithospheric and/or crustal diapir emplacement.
To conclude, a synergic/multidisciplinary approach, set at the boundaries of different research fields (i.e. geophysics, soft condensed matter, fluid dynamics), and the combination of complementary methodologies (i.e. laboratory and numerical modeling, rheometry, analysis of natural data) are necessary to understand complex phenomena occurring in a planetary system.
Fluides & Matériaux Complexes
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