Wetting and drying in nanometric pores : thermal effects and challenges in the field of energy

Confined water plays a crucial role in the biochemical processes of living organisms, but also in various phenomena at the nanometric scale that are fundamental to addressing several major societal challenges, such as freshwater supply, which is ensured in many parts of the world by desalinating seawater, or the defossilization of our energy resources by harnessing the mixing of fresh water from rivers with seawater in estuaries. This non-intermittent energy source, known as osmotic energy¹, has been listed in the European Renewable Energy Directive since 2022.

In this context of water and energy applications, fundamental research in nanofluidics aims to understand coupled transport phenomena, mainly mass, solute, and ion current transport at the nanoscale. However, heat transport and, more broadly, the specific thermal effects at these scales have remained in the shadows, even though their role is significant. This is the context for this thesis project, funded by the Franco-Brazilian ANR project HTD-PoM, which aims to explore the mechanical and thermal phenomena involved in the confinement of aqueous solutions in hydrophobic nanopores. Three areas of focus are envisaged for this experimental work: the study of the subtle coupling between osmotic energy and thermal energy, the characterization of the thermoelastic properties of confined fluids, and finally the exploration of the thermal conductivity of nanoporous materials as a function of their content. Certain nanoporous materials are exceptional thermal insulators.

However, we recently predicted a surprising behavior numerically: even lower thermal conductivity when the particles are filled with liquid, contrary to what is observed on a macroscopic scale (everyone knows that wet clothing is less insulating and promotes heat transfer). Does the experiment confirm this prediction? Your thesis will help answer this question.

More specifically, the work will be based on an original experimental approach using ordered hydrophobic nanoporous particles immersed in water or an aqueous solution. Forced filling is achieved by pressurizing the liquid to several hundred bars, while drying occurs spontaneously when the pressure is released. The experimental setup allows for detailed characterization of the pressures and temperature variations characteristic of these processes as a function of their duration, which is controlled by the setup over four decades of time². These mechanical and thermal measurements constitute macroscopic signatures of the wetting and drying phenomena at work under nanometric or even sub-nanometric confinement, phenomena whose study has remained marginal in the field of nanofluidics.

This thesis will be carried out in conjunction with numerical work as part of the HTD-PoM project and will also contribute to an applied project at the laboratory dedicated to an innovative approach to osmotic energy recovery³. Prerequisites include a solid background in physics, soft matter, and thermodynamics, as well as a keen interest in physical chemistry and, more generally, an appetite for interdisciplinarity, instrumentation, and experimental challenges!

Application: https://emploi.cnrs.fr/Offres/Doctorant/UMR5588-CYRPIC-003/Default.aspx?lang=EN

1. La Science, CQFD – Energie osmotique, ce rêve bleu, Quoi de neuf chercheur – L’Énergie Bleue expliquée par Cyril Picard.
2. L. Michel et al. A Dynamical calo-porosimeter to characterize wetting and drying processes in lyophobic nanometric pores, Rev. Sci. Instrum. 95.10 (2024),
3. C. Picard et al. Procédé de conversion d’énergie osmotique en énergie hydraulique et de dessalement.