The partitioning of the fluid phase into a biphasic air-water assemblage characterizes the unsaturated zone. The properties of the retained liquid are depending on the intensity of the capillary forces which are linked to the pore size hosting the capillary bridges. In this experimental study, we are scrutinizing the timeline, the liquid volume, the water-gas-mineral interactions, which may be impacted by the capillary conditions.

Experiments were developed using synthetic bimodal pore system to created capillary conditions affecting large volume of trapped water (dual-pores geometry). The chosen materials (silica microcapillaries, simple supersaturated solutions) and methods (optical microscopy) enabled direct long term observations inside the pore of systems under close-to-equilibrium conditions, and made it possible to provide a thermodynamic interpretation of the water-mineral-gas interactions.

First, capillary water has a higher solubility for gases and minerals, making it a super solvent analogous to a supercritical liquid. Additionally, immobile solution in capillary pores can reach a larger chemical saturation than within the mobile solution present in neighbouring larger pores. Second, the heterogeneity of a dual pore system makes it prone to cavitation processes that cancel the capillary conditions after a time. However, we show evidences that capillarity can be regenerated in cavitated pore body, if drying conditions remained constant at the pore scale. This "self-healing" mechanism promotes a very long lifetime of capillary conditions in the presence of adequate pore structures. Chemical and mechanical processes that are specific to capillary conditions should then be expected in the long-term dynamics of unsatured zones.