Ph-D in Geodynamics (M/F): Convection in diphasic fluids, from the laboratory to planetary interiors

Job Description

Thesis Subject

Rayleigh-Bénard convection develops when a plane layer of fluid is heated from below and cooled from above. Ubiquitous for mass and heat transfers in many industrial systems, it has also been identified as a key-player in the dynamics of the interior of stars and planets, and of the oceans and the atmosphere. A large body of theoretical and experimental work exists for fluids with constant viscosity. However, for rocky planets and icy satellites, the variation of temperature can induce arrest and/or gelation of their microstructure and a drastic increase of their viscosity. So convection develops in a sublayer under a lid or skin (what is called lithosphere in a planet), that takes up most of the viscosity variation. This lid remains stagnant and strongly limits heat and mass transfer across the surface. However, the skin very often has also solid-like properties and can develop shear banding, wrinkles and even cracks. In addition it can contains fluids like brines or melt. This texture at the meso-scale will affect the skin interaction with convection and the large-scale dynamics of the layer. These in turn will affect skin formation and texture. This interplay of the micro-meso-macro structures govern the evolution of rocky planets. Plate Tectonics on Earth, whereby 60% of the Earth surface is continuously renewed, is an exemple of the skin breaking and sinking back in the convective layer. But it is only one convective pattern/regime among many.
Our group has developed a long-term program of experimental studies on convection in complex fluids. The use of soft matter materials such as colloidal dispersions allows to study convection as a function of the fluid rheology (from viscous to visco-elasto-plastic to brittle), and to probe the fluid texture
(elementary organization, shear banding, faults, ...) at all scales from the nm to the convection scale (cm-m). So far, silica aqueous colloidal dispersions are the only laboratory fluids capable of producing the sufficient shear localization to generate self-consistently asymmetric subduction (fig.1b) and the morphology of mid- ocean ridges (transform faults, overlapping spreading centers, microplates). Interestingly enough, subduction is observed for experimental lithospheres still containing a little of free liquid. This suggests that water oceans or melt could be important to allow subduction on a rocky planet.
So we propose to investigate during this Ph-D thesis the interplay between convection and two-phase flow using state-of-the-art laboratory experiments, scaling analysis and numerical simulations. The objectives are: 1) to determine the phase diagram of the different regimes of convection (e.g. heat pipes, stagnant lid, plate tectonics, local surface rejuvenation, catastrophic global rejuvenation, new regime,...) ; 2) to characterize each convective regime; 3) to understand the influence of the biphasic nature of the fluid on the convective patterns; 4) to use these results to constrain the evolution of planets and especially their inner mantle dynamics. Of particular interest will be Venus, that the ESA EnVision and the NASA VERITAS missions should reach in the early 2030s.

Your Work Environment

This 3-year Ph-D project is funded through the European Research Council Advanced Grant SOFT- PLANET. SOFT-PLANET involves a pluridisciplinary (soft matter physics, fluid mechanics, geodynamics, planetology) and international team to study the different regimes of convection and surface patterns formation in complex fluids, and thereby decipher the current state and evolution of planets. The research will be done in Laboratoire FAST, a Fluid Mechanics department of CNRS and Université Paris-Saclay.

Constraints and risks

no particular risks for this project

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

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