Turbulent transport in an idealized patch of ocean

The ocean absorbs 90% of the heat associated with global warming and 30% of anthropogenic CO2. How such tracers are accumulated and redistributed within the turbulent ocean is a central issue of long-term climate prediction. The challenge stems from the existence of ocean mesoscale eddies: turbulent vortices tens of kilometers wide that are not resolved by climate models despite being key contributors to ocean transport. A way forward consists in leveraging the scale separation between the mesoscale and the extent of ocean basins. A multiscale expansion indicates that the transport properties of the ocean can be determined from the study of a local “patch” of ocean, before being implemented in a large-scale model.

The goal of the PhD project is thus to determine the turbulent transport properties of a realistic local patch of ocean, based on a hierarchy of models: single-layer models, multi-layer models and fully three-dimensional models, on the f-plane and on the beta-plane. Various ingredients of realistic flows will be considered, such as the influence of bottom topography and the interaction between balanced flows and wave modes. Time-permitting, the resulting transport coefficients will be implemented in global models of increasing complexity.

Multiscale metamaterials based on 3D-printed biosourced polymer composites

Reducing the density of materials is one of the best ways to diminish our energy footprint. One solution is to replace massive materials by microlattices. Among these, random architecture structures inspired by bird bone structure offer the best advantages, with isotropic mechanical behavior and increased mechanical resistance, while meeting the challenges of the circular economy. These material-saving metamaterials are manufactured by 3D printing and can be compacted at the end of their life cycle. Among manufacturing technologies, UV polymerization of liquid organic resin or composite is the most promising. It produces mechanically resistant materials without generating manufacturing waste. It is also possible to include large quantities of bio-sourced fillers, reducing even further their environmental impact.

The PhD-thesis proposed here focuses on the development of polymeric nanocomposite microlattice structures from resin formulation to mechanical properties study (viscoelasticity, yield stress, fracture resistance) through printing and post-processing stages. From a more fundamental point of view, the aim is to study the link between the composition, shape and surface properties of the fillers on one hand, and the imprimability of the composite resine and the mechanical properties of the resulting metamaterial on the other hand. The thesis will focus on the study of cellulose-type fillers in nanoparticle, microparticle or fiber form. This multidisciplinary study bridges technology to science while producing data for a digital twin.

Physico-chemical multi-scale modelling of coalescence

The recycling of metals by hydrometallurgy is necessary to ensure both the development of new energy technologies and the sustainability of the nuclear power cycle. The industrial processes used here, such as liquid-liquid extraction, involve the flow of two fluids under agitation whose interfaces form and deform. The coalescence of drops thus plays a very important role. The modelling of these complex two-phase systems involved in the extraction process must therefore take this phenomenon into account in the best possible way. In this thesis, we propose to describe for the first time the coalescence of drops in a realistic way by a multi-scale approach to take into account the physicochemical aspects of the phenomena. Firstly, molecular dynamics simulations will allow us to propose a stochastic model where the probability of coalescence will be expressed as a function of geometric parameters (distance and size of drops). Then the role of physico-chemical quantities in the phenomenon will be systematically described (role of surface tension, presence of surfactants, viscosities, etc.). The ultimate goal is to arrive at computational fluid dynamics (CFD) simulations in which coalescence, and in particular its random nature, will have been developed and validated by molecular dynamics calculations performed at the atomic level, taking into account the physicochemistry of the interface. Such a model would be a major step towards predicting the kinetics of liquid two-phase media, not only in the context of optimising recycling processes, but also for many other applications.
Candidate profile: Master of Physics - Master of Chemistry - Engineering School - ENS. After the thesis, the candidate can either continue in the academic field, with a high level of competence in modelling, or move towards industry by developing his/her dual competence in modelling and recycling.

Nano-object simulations in biological media

Understanding the non-specific or specific interactions between biomolecules and nanomaterials is key to the development of safe nanomedicines and nanoparticles. Indeed, adsorption of biomolecules is the first process occurring after the introduction of biomaterials into the human body, which controls their biological response. In this thesis, we will simulate the interface between nanosystems and biomolecules on a scale of a hundred nanometers, using the new exascale computing resources available at the CEA from 2025 (Jules Verne machine installed at the CCRT).

Novel membranes based on 2D nanosheets

This thesis project aims to exfoliate new nanostructured architectures based on two-dimensional inorganic phases. These nanostructures will be designed for filtration devices and tested using our microfluidic platform. The target application is water purification and the selective separation of metal ions. The doctoral student will interact with chemists, physicists and electrochemists in a real multidisciplinary environment, on a fundamental research subject directly connected to application needs. Thus, during his thesis, the student will be exposed to a multidisciplinary environment and brought to carry out experiments in various fields such as inorganic chemistry, physical chemistry, micro / nano-fabrication and nano-characterization methods. In In this context, this project should potentially lead to significant societal benefits.

For the realization of the latter, he will have access to a very wide and varied range of equipment ranging from optical microscopes to the latest generation synchrotron (ESRF), including field effect or electron microscopes and galvanostats.

This thesis is therefore an excellent opportunity for professional growth, both in terms of your knowledge and your skills.

Thermoelectric energy conversion in nanofluids for hybrid solar heat collector

Thermoelectric (TE) materials that are capable of converting heat into electricity have been considered as one possible solution to recover the low-grade waste-heat (from industrial waste-stream, motor engines, household electronic appliances or body-heat).

At SPHYNX, we explore thermoelectric effects in an entirely different class of materials, namely, complex fluids containing electrically charged nanoparticles that serve as both heat and electricity carriers. Unlike in solid materials, there are several inter-dependent TE effects taking place in liquids, resulting in Se values that are generally an order of magnitude larger that the semiconductor counterparts. Furthermore, these fluids are composed of Earth-abundant raw materials, making them attractive for future TE-materials that are low-cost and environmentally friendly. While the precise origins of high Seebeck coefficients in these fluids are still debated, our recent results indicate the decisive role played by the physico-chemical nature of particle-liquid interface.

The goal of the PhD project is two-fold :
- First, we will investigate the underlying laws of thermodynamic mechanisms behind the thermoelectric potential and power generation and other associated phenomena in nanofluids. More specifically, we are interested in how the particles' Eastman entropy of transfer is produced under the influence of thermal, electrical and concentration gradients. The results will be compared to their thermos-diffusive and optical abosrption properties to be obtained through research collaborations.
- Second, the project aims to test the promising nanofluids in the proof-of-concept hybrid solar-collector devices currently developed within the group to demonstrate the co-generation capability of heat and electricity. The hybrid device optimization is also within the project's scope

The proposed research project is primarily experimental, involving thermos-electrical, thermal and electrochemical measurements; implementation of automated data acquisition system and analysis of the resulting data obtained. The notions of thermodynamics, fluid physics and engineering (device) physics, as well as hands-on knowledge of experimental device manipulation are needed. Basic knowledge of optics and electrochemistry is a plus. For motivated students, numerical simulations using commercial CFD software, as well as the optical absorption measurements at the partner lab (LNO/CNR, Florence, Italy) can also be envisaged.

Rheology of partially-crystallized glass melts: from experimental data acquisition to modeling

The formulation of a radioactive waste packaging glass is the result of a compromise between waste loading, technological feasibility and chemical durability. Maximizing the waste loading rate allows us to reduce the number of vitrified packages produced, and consequently the volume and cost of the underground disposal. On the other hand, increasing this loading rate beyond a certain threshold is likely to lead to the presence of crystals in the vitrification furnaces. However, such an evolution of glass formulations requires, among other things, verification of the impact of the presence of these crystals on the properties of the glass during its production (at 1100-1200°C), in particular its rheology, a key property for the good operation of vitrification furnaces. The aim of the proposed thesis is therefore to measure and then model the effect of crystals on rheology, as a function of time, temperature, and nature and morphology of the crystals, and to take into account the risk of sedimentation. To do this, experimental data will have to be acquired, then modelled using models proposed in the literature, which may need to be adapted. A Master or engineering degree in physico-chemistry or material sciences is needed.

Dielectric response of a liquid far-from-equilibrium

Materials in the glassy state are of great practical interest and can be found in many applications: silica glass as a construction or transport material, plastics which are generally at least partially glassy, or glassy metal alloys for advanced applications. However, the physical properties of these materials (e.g. the strength of a telephone screen) depend on the heat treatment they receive during their formation, and more specifically on the rate of cooling from the liquid state. While industrial glass manufacturing processes are obviously well mastered, the non-equilibrium thermodynamic nature of these systems makes it particularly difficult to investigate the physical mechanisms at work theoretically and numerically. This calls for an experimental approach aimed at probing these fundamental mechanisms.

The aim of this PhD thesis is to study experimentally the very non-equilibrium response of polar liquids, using a device recently developed in the laboratory which enables us to apply a very rapid temperature change to a liquid and follow its re-equilibration dynamics. Measurements of linear response should reveal more about the physical mechanisms governing equilibration, while non-linear measurements will provide information about the cooperative nature of structural rearrangements.

Impact of ultrasound on the flow properties of complex suspensions

Nuclear industry generates radioactive wastes of various nature such as solids, liquids but also sludges coming from effluent treatment facilities or historical residues stored in pool or tanks. The physico-chemical nature of those sludges leads to a complex flow behaviour making it difficult to handle and convey prior their immobilization in a conditioning matrix. In order to fluidize these suspensions of varying compositions, the mechanical action of power ultrasound is envisaged. It has recently been shown, thanks to a set-up coupling power ultrasound and rheology, that it is possible to significantly reduce the yield stress and viscosity of the slurry by applying ultrasound. The aim of this thesis is to pursue the studies already undertaken (physical chemistry, microstructure, ultrasound and rheology) on reconstituted sludge or simplified model suspensions, focusing more specifically on two aspects. The first, more fundamental, will aim to gain a better understanding of the interaction between power ultrasound and matter, with a particular focus on the origin of the effects observed (interfaces vs. volume). The second aspect will be more applied, with the development of original larger-scale experimental devices capable of generating flows closer to industrial situations. For this phD work, we are looking for a motivated, serious and curious candidate. Given the multidisciplinary character of the subject, mixing physics, physico-Chemistry and experimental development, the candidate could valorize his new skills in various industrial fields such as nuclear, civil engineering and depollution domain.

Experimental study of boundary layers in turbulent convection by Diffusive Waves Spectroscopy

The aim of this thesis is to carry out the first experimental measurement of the energy dissipated in the boundary layers during turbulent convection in the Rayleigh-Bénard configuration. Indeed, some theories assert that this quantity controls the heat flux transported from the hot wall to the cold wall, while the efficiency of turbulent transport in convection is the subject of debate. Yet the properties of turbulent transport are essential to understanding the dynamics of climate and many astrophysical objects.

To estimate the energy dissipated, we need to be able to measure the norm of the velocity gradient. This quantity is difficult to access with conventional anemometry techniques, which measure velocity fields with limited resolution. These gradients are also expensive to obtain numerically over long time scales. But we have developed a technique for directly measuring the norm of velocity gradients using Multiple Scattering Spectroscopy. This will enable us to measure dissipative structures and the rate of energy dissipation in boundary layers.

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