Capillary-driven displacement of immiscible fluid within woven fibrous porous media by permeating liquid.

HYPOTHESIS: Permeation of viscous fluid in porous media has been attracted strenuous attention to comprehensively understand liquid-gas and liquid-liquid exchanges in a range of industrial fields as well as in nature. Precise prediction of immiscible-fluid displacement has not been made, however, even under viscous- and capillary- dominant or low Reynolds (Re) and capillary (Ca) number conditions, nevertheless localized interfacial events near the advancing flow fronts must be governing events within porous media with geometric anisotropy.

EXPERIMENTS: Displacement of immiscible gas induced by viscous liquid is targeted by considering a geometry of a single-layer woven-fibrous porous medium in good wetting condition confined between flat and smooth parallel plates. Via experiments, fine controlled pressure reduction within the geometry induces viscous liquid permeation under designated pressure differences. Numerical simulation, validated by the present experiments, are also conducted by applying 'volume of fluid' and 'continuum surface force' methods to measure the velocity and pressure fields in well-reproduced permeation and resultant displacement of immiscible fluid.

FINDINGS: Twofold successive snap-off events in elongated immiscible fluid 'throat' trigger the formation of an isolated bubble, or void, entrapped in the inter-bundle region. Slight increment in Re and Ca, even though under Re ≪ 1 and Ca ≪ 1, results in only a single or no snap-off event due to the effect by the Young-Laplace pressure, leading to perfect displacement of the immiscible fluid sucked by their bulk downstream. Such sensitive reaction against Re and Ca is demonstrated to induce a drastic variation of macroscopic void fraction in the target geometry.