Speaker
Description
In this study, we investigate the interplay between capillarity, gas dissolution and salt crystallization (capillarity-crystallization dynamics) within novel, reproducible, and depth-variable (2.5D) polydimethylsiloxane (PDMS)-glass microfluidic channels with controlled submicron features. These channels more accurately replicate varied pore throat morphologies found in geologic and other porous media than traditional 2D microfluidics. Capillarity-crystallization dynamics are studied within the presented 2.5D nano/microfluidic channels through imbibition and trapped bubble experiments for varied fluid salinites.
The 2.5D microfluidic chips are fabricated by casting PDMS on a reusable mold produced by direct laser writing (a 3D-printing technique that uses a focused laser to polymerize microstructures), followed by bonding the PDMS to glass via oxygen plasma. The chips feature an array of converging/diverging channels of rectangular, triangular, and semicircular cross sections that emulate idealized granular pore “throats” in conventional sandstone rocks, as well as straight channels of different cross sections to represent idealized grain boundaries and microfractures in dual-porosity media such as basalts and other mafic/ultramafic rocks. The smallest constrictions are 1 μm in depth and 2 μm in width in various cross sections, highlighting the fabrication technique’s resolution. Additionally, the technique results in regular, nanoscale surface steps within the channels that are a function of the 3D-printing settings, allowing an additional control over surface geometry. The chips are designed such that during imbibition, gas is trapped inside each channel. Gas-liquid interfaces and any crystallized mineral-liquid interfaces in each channel are captured with optical microscopy. Image analysis enables quantification of changes in gas dissolution, brine-gas interfacial area, and mineral precipitation location/geometry. The controlled channel geometry allows for comparison of capillarity-crystallization dynamics to analytical frameworks that integrate capillary pressure, viscous losses, gas partitioning (Henry’s Law), and mass transport. The presented lab-on-a-chip platform and analysis scheme enables effective gas-liquid transfer (bubble dissolution) and crystal growth rates to be calculated as a function of precision pore geometry, surface properties, and fluid salinity.
We find that 2.5D cross-sectional geometry and channel convergence strongly influence bubble dissolution rate and secondary phenomena such as condensation (within the trapped bubble) and salt crystallization, particularly when comparing channels with higher-curvature menisci due to corners (i.e., triangular cross section). The initial findings are compared to 2D microfluidic (base case) geometries with uniform cross sections. We identify where 2D limitations can bias flow patterns and the intensity of capillary pressure effects, reducing their applicability to real-world porous media. Going forward, our work will vary other fluid parameters (pH, gas type), continue to explore other geometric proxies of subsurface microstructures, and further compare the capillary-crystallization data to new analytical frameworks and theoretical models. Outcomes will enable tuning of brine composition based on subsurface matrix geometric contractions to optimize and better predict subsurface brine-gas partitioning and geochemical transport processes.
| Country | United States |
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