Search Results (3)

This work examines a type of rapid pore-filling event in multiphase flow through permeable media that is better known as Haines Jump. While existing microfluidic experiments on Haines Jump mostly seek to maintain quasi-steady states through very low bulk flow rates over long periods of time, this work explores the combined use of a highly structured microscale transport network, high-speed fluorescent microscopy, displacement front segmentation algorithms, and a tracking algorithm to build evolution graphs that track displacement fronts as they evolve through high-speed video recording. The resulting evolution graph allows the segmentation of a high-speed recording in both space and time, potentially facilitating topology-cognitive computation on the transport network. Occurrences of Haines Jump are identified in the microfluidic displacement experiments and their significance in bulk flow rates is qualitatively analyzed. The bulk flow rate has little effect on the significance of Haines Jump during merging and splitting, but large bulk flow rates may obscure small bursts at the narrowest part of the throat. Full article

Microgravity hinders capillary-driven water flow in unsaturated porous media. Previous studies proposed pore-scale phenomena such as “air entrapment”, “particle separation”, and “interruption on widening void space” to explain gravity-dependent capillary-driven flows. Our objectives were: (1) to measure the water flux densities of the pore-scale capillary-driven flow in micro- and Earth-gravities and (2) to reveal that what makes water flow slower under microgravity than under 1 G. We found that average macroscopic water flux densities had no significant difference under micro- and Earth-gravities (p = 0.30). We did not observe “air entrapment” in the pore spaces of porous media. “Widening on a single particle” and “capillary widening” disturbed capillary-driven flow; however, “widening on a single particle” had no significant gravity dependency. “Capillary widening” may be independent of gravity, since it was observed both under microgravity and under 1 G. Water flux densities in unsaturated porous media may have gravity dependency induced by “particle separation” only when porosity is large enough to allow particles to move. Full article

Understanding the mechanisms involved in the formation and growth of methane hydrate in marine sandy sediments is crucial for investigating the thermo-hydro-mechanical behavior of gas hydrate marine sediments. In this study, high-resolution optical microscopy and synchrotron X-ray computed tomography were used together to observe methane hydrate growing under excess gas conditions in a coarse sandy sediment. The high spatial and complementary temporal resolutions of these techniques allow growth processes and accompanying redistribution of water or brine to be observed over spatial scales down to the micrometre—i.e., well below pore size—and temporal scales below 1 s. Gas hydrate morphological and growth features that cannot be identified by X-ray computed tomography alone, such as hollow filaments, were revealed. These filaments sprouted from hydrate crusts at water–gas interfaces as water was being transported from their interior to their tips in the gas (methane), which extend in the µm/s range. Haines jumps are visualized when the growing hydrate crust hits a water pool, such as capillary bridges between grains or liquid droplets sitting on the substrate—a capillary-driven mechanism that has some analogy with cryogenic suction in water-bearing freezing soils. These features cannot be accounted for by the hydrate pore habit models proposed about two decades ago, which, in the absence of any observation at pore scale, were indeed useful for constructing mechanical and petrophysical models of gas hydrate-bearing sediments. Full article