PhD defense Viktor Gredicak

Abstract

This PhD thesis is part of the INTER-AQ project within CNRS, an interdisciplinary research initiative aiming to unravel the complex and coupled dynamics of mass transfer and fluid redistribution at fluid-fluid interfaces within porous media. The project combines experimental microfluidics to replicate subsurface environments, vibrational spectroscopy for in situ chemical quantification, and novel plasma-based techniques to tailor pore surface properties. These integrated approaches target key environmental and energy challenges, such as geological CO2 storage. Within this framework, the PhD research contributes to a pore-scale perspective, advancing experimental methods to investigate how interfacial phenomena influence fluid behavior in porous systems.

The objective of this thesis is to address a fundamental gap in our understanding of mass transfer at the pore scale, specifically how wettability affects fluid/fluid interfacial mass transfer processes and fluid trapping in geological porous media. The research focuses on the development of an experimental platform that integrates microfluidics and μRaman spectroscopy to track CO2-water interfacial mass transfer in real time. Microfluidic devices are transparent pore networks made of channels of rectangular cross-sections, mimicking subsurface porous environments and allowing direct visualization of fluid behaviors. Crucially, the study introduces in situ atmospheric pressure plasma treatment as a new tool for modifying wettability within closed glass microfluidic devices. This allows precise control over surface characteristics, enabling systematic investigation of wettability impacts on interfacial dynamics. The insights of wettability influenced interfacial mass transfer gained in this study are expected to improve the calibration of both pore- and field-scale models, increasing the accuracy of predictions for carbon storage in geological reservoirs.Three core research questions structure the investigation. First, the feasibility of propagating atmospheric plasma within closed microfluidic channels is explored to determine whether in situ wettability tuning is technically viable. The work demonstrates that plasma jets can be successfully introduced and controlled within the devices, enabling reliable alteration of surface wettability. Results obtained by in situ contact angle measurement on images indicate uniform wettability treatment with increased hydrophilic properties after only 1 min of plasma treatment. The wettability achieved on glass with our setup offers stability for up to 70 days, depending on the plasma treatment and storage parameters. Second, the study examines how wettability conditions affect capillary trapping and interfacial mass transfer. By employing microfluidic devices with increasing geometrical complexity, the experiments reveal how surface wettability indeed influences fluid displacement/ retention, water evaporation and CO2 dissolution under a range of flow conditions. Our results demonstrate the influence of wettability on fluid-fluid displacement and mass transfer processes. Third, the potential of μRaman spectroscopy for in situ, semi-quantitative analysis of CO2 dissolution is assessed. Raman analyses is able to capture the temporal evolution of dissolved species, providing complementary chemical data to visual flow observations and enabling the quantification of interfacial mass transfer rates.

In summary, this research establishes an integrated atmospheric-plasma-microfluidic-μRaman experimental framework that allows for the systematic study of pore-scale interfacial mass transfer under controlled wettability conditions. By leveraging plasma technology for surface modification and spectroscopic techniques for chemical analysis on microfluidic devices, the thesis delivers new insights into the mechanisms that govern CO2 trapping in porous media and that are critical for the design and optimization of carbon sequestration strategies.