This proposal aims to develop a combination of a chirped-pulse (sub)mm-wave rotational spectrometer with uniform supersonic flows generated by expansion of gases through Laval nozzles and apply it to problems at the frontiers of reaction kinetics. The CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique, combined with laser photochemical methods, has been applied with great success to perform research in gas-phase chemical kinetics at low temperatures, of particular interest for astrochemistry and cold planetary atmospheres. Recently, the PI has been involved in the development of a new combination of the revolutionary chirped pulse broadband rotational spectroscopy technique invented by B. Pate and co-workers with a novel pulsed CRESU, which we have called Chirped Pulse in Uniform Flow (CPUF). Rotational cooling by frequent collisions with cold buffer gas in the CRESU flow at ca. 20 K drastically increases the sensitivity of the technique, making broadband rotational spectroscopy suitable for detecting a wide range of transient species, such as photodissociation or reaction products. We propose to exploit the exceptional quality of the Rennes CRESU flows to build an improved CPUF instrument (only the second worldwide), and use it for the quantitative determination of product branching ratios in elementary chemical reactions over a wide temperature range (data which are sorely lacking as input to models of gas-phase chemical environments), as well as the detection of reactive intermediates and the testing of modern reaction kinetics theory. Low temperature reactions will be initially targeted; as it is here that there is the greatest need for data. A challenging development of the technique towards the study of high temperature reactions is also proposed, exploiting existing expertise in high enthalpy sources.

Understanding and modelling reactive transport in porous media is fundamental to predicting field-scale biogeochemical reactions, which play a key role in current environmental issues such as water resources management and carbon dioxide sequestration. A major scientific challenge is to capture the dynamics of coupled solute mixing and reaction processes in the context of multiscale heterogeneity, which characterise most natural porous media. In particular, the impact of pore-scale mixing on large- (Darcy-)scale reactive transport is a critical scientific question. ChemicalWalks addresses this question by coupling for the first time the lamella theory of mixing, developed by the host supervisor, and the chemical CTRW model for reaction kinetics under incomplete mixing, recently developed by the ER. While the lamella theory has successfully quantified mixing processes and fluid-fluid reactions at pore scale, its application to fluid-solid reactions, which are ubiquitous in natural systems, remains to be explored. The key idea of ChemicalWalks is to use the lamella theory to determine how pore-scale concentration distributions control the distribution of fluid-solid reaction rates, and formalize a predictive theory for upscaled reaction kinetics through the chemical CTRW framework (WP1). The complementary expertise of the researcher and the host will ensure a particularly efficient two-way transfer of knowledge to achieve this goal. This will open the door to the development of a hybrid computational method, quantifying the effect of pore-scale mixing on Darcy-scale reactive transport phenomena at a scale relevant to environmental applications (WP2). ChemicalWalks will be firmly rooted on a career development plan and supported by scientific training in state-of-the-art mixing theories and data processing and interpretation techniques, placing the fellow at the forefront of reactive transport modelling.

Solute transport in unsaturated porous media plays a crucial role in environmental processes affecting soils, aquifers, and carbon capture and storage operations. Natural porous media are characterized by various degrees of structural heterogeneity in the pore size distribution, spatial arrangements and spatial correlations. The impact of this pore-scale heterogeneity on the spreading of a solute plume, its mixing with other solutes, and the resulting reaction rates, is not well understood for unsaturated flow. Since these processes take place at pore scale, direct pore scale experimental measurements are needed to gain comprehensive understanding of them. The aim of UnsatPorMix is thus to elucidate the impact of structural pore-scale heterogeneity on solute spreading/mixing and reaction rates during unsaturated flow, through the combination of micromodel experiments and numerical model simulations. In the first stage of UnsatPorMix, experiments in micromodels with varying degrees of heterogeneity will provide unprecedented results on the phenomenology of pore-scale mechanisms and their effect on solute spreading and mixing. In the second stage, the experimental measurements of phase distribution and solute concentrations, combined to numerically-computed pore scale velocities, will be used to design and validate a pore-scale model for solute transport in these porous media. This model will allow obtaining a large representative numerical data set, enabling statistical analysis and the derivation of quantitative relations between structural heterogeneity and solute transport/mixing. UnsatPorMix will make a significant contribution to the modelling of, and risk assessment for, the various subsurface phenomena and applications cited above. During UnsatPorMix, the applicant will acquire a set of invaluable experimental skills and modeling expertise which will enable him to become an independent researcher and expert in flow and transport in unsaturated porous media.

Source–to-sink (S2S) systems describe the response of the Earth's surface to tectonic and climatic signals over geological times, from upstream zones of erosion (source) to ultimate deposition (sink). Understanding S2S systems is key to significantly improve human's ability to predict the characteristics of sedimentary accumulations hosting essential societal and industrial resources (geomaterials, energy, minerals, groundwater, waste). Tomorrow’s successful exploration of Earth's resources for future sustainable and responsible growth relies on training the next generation of researchers with a holistic and interdisciplinary approach to S2S systems. The S2S-FUTURE project, closely co-designed by academics and professional partners to enhance societal relevance and employment demand, has 3 major ambitions: - Train 15 ESRs with the highest level of state-of-the-art concepts, techniques, and multidisciplinary knowledge to comprehend complex large-scale S2S systems with creativity, innovation and numerical modelling tools. - Empower these ESRs with acute awareness of career opportunities in both academic and non-academic sectors thanks to selected entrepreneurial skills and involvement into a diverse professional network of world leaders in the domain. - Promote the efficiency of European S2S research and application to meet society needs through the establishment of a dynamic network of professional and the development of legacy tools that will ensure the network's perennity and its expansion beyond this ITN: a yearly S2S Summer School and a EU web-portal for S2S. Europe and the S2S-FUTURE trained ESRs will be at the center of this network, in a unique place to contribute and benefit from future capacity building and breakthroughs of source-to-sink sedimentary research.

Subsurface reactive processes play a key role in dictating the evolution of subsurface environments, their interaction with surface water bodies and the migration and remediation of transported contaminants. In particular reactive hot spots tend to concentrate in mixing fronts between fluids of different compositions, such as recently infiltrated/injected fluids and resident groundwater, which develop in a range of situations, including CO2 sequestration operations and geothermal systems, contaminant remediation operations, and reactive hyporheic zones beneath rivers. Our understanding of the development and temporal dynamics of these hotspots is currently hampered by the limited sampling offered by boreholes. Recent breakthroughs in geoelectrics may however profoundly change our vision of these phenomena by providing non-invasive techniques with high sensitivity to many geological processes. GeoElectricMixing will hence develop a novel approach to investigate the temporal dynamics of reactive mixing processes from Complex Impedance and Self Potential signals. The coupling of reactive mixing and geoelectrics will be quantified and upscaled by integrating charge transport and polarization phenomena in a new modeling framework, recently developed by the host to predict the spatial distribution of chemical species and reaction rates across mixing fronts (WP1). Dedicated experiments will then be designed by integrating electrodes in a novel millifluidic setup to monitor jointly the temporal evolution of geoelectrical parameters and the spatial distribution of concentrations and reactions rate in a reactive mixing front progressing through the cell (WP2). GeoElectricMixing is thus expected to open a new window on subsurface reactive mixing phenomena, expanding our capacities to detect and quantify these processes in situ, and thus providing critical data to unlock current open questions on the dynamics of mixing processes and their role in reaction enhancement.