Chemo-mechanical coupling at the one-pore scale: fracturing quartz host by increasing tension in water inclusion
Résumé
Water-bearing porous media allow the infilling liquid to become superheated much easily than any
other system. Superheating liquids make them prone to develop a tensile state, that is to say an internal
negative liquid pressure or tension. In this sense, superheated liquid is a close analogue to capillary
water retained in many not-saturated porous media (unsaturated zone of soils, gas/oil-depleted
aquifers, CO2-storing aquifers). In granular physics, the role of capillary water tension to rigidify the
granular assemblage is studied for long (sand–castles physics) but little attention has been turned to
the same effect in compacted/continuous systems (rock fissures, fluid inclusions, intra-mineral
cavities, etc.).
Using synthetic fluid inclusions trapped in quartz, we were able to put the occluded liquid (aqueous
solution, CsCl 12m) at very high tension by isochoric cooling of the samples. Starting with a
liquid-vapour assemblage, we heat the sample up to a special temperature at which the vapour bubble
disappears (temperature of homogeneization, Th), and then turn to a cooling procedure that decreases
the internal pressure of the occluded liquid at constant volume, as long as the bubble does not
re-appear again (relaxing the tensile state of the liquid). At a given tension state, we mapped the
Raman spectra at the two quartz bands frequencies, in the quartz matrix all around the inclusion under
tension. Using frequency-pressure calibration of the literature, it turned out that the quartz host was
submitted to a small compressive stress in response to the perpendicular traction from the liquid.
In a second step, one sample was submitted during one month to repetitive cycles of
superheating-relaxation processes, after which the volume of the inclusion changed brutally. This was
recorded by a change of the liquid density measured through a significant Th shift. In the meantime,
another sample was submitted to a constant tension which, after a while, provoked the visible
fracturation of the quartz matrix.
These observations and measurements demonstrate that the tension of water occluded in pores, or
channels, or any type of cavities in solids, organics or living cells, is able to exert a stress onto the host
solid, quantitatively weak. However, it appears that the recurrence of such effect and/or its
preservation through time, create a fatigue in the host that is ultimately able to break out its cohesion,
certainly owing to pre-existing matrix defaults. Consequences in terms of chemo-mechanical coupling
in hydrosystems or materials submitted to wetting-drying cycles will be eventually highlighted.