Unveiling nanoscale fluid miscible behaviors with nanofluidic slim-tube

Fluid miscible behaviors in nanoporous media are crucial for applications such as carbon capture, utilization, and storage (CCUS), membrane separation, subsurface pollutant remediation, and geothermal extraction. Confinement effects at the nanoscale cause fluid miscible behaviors to deviate from bulk phases, and the underlying mechanisms remain inadequately understood. Here, we developed a nanofluidic slim-tube method to directly visualize fluid miscible behaviors and measure the minimum miscibility pressure (MMP) at the nanoscale. Focusing on CO2-hydrocarbon systems—an intersection of low-carbon energy transition and environmental sustainability—we investigated miscibility within multiscale porous media featuring pore sizes from 100 nm to 10 μm for the first time. Our results demonstrate that in nanoporous media, the CO2 diffusion front advances faster than the displacement front, indicating that molecular diffusion dominates mass transport. Miscible flow reduces CO2 fingering caused by mobility differences, achieving ~100% displacement efficiency. In multiscale porous media, distinct miscible stages emerge due to fluid composition variations at different scales and CO2 selective extraction. Our experimental findings also reveal that MMP decreases in nanoporous media compared to bulk values. However, in multiscale porous media, MMP exceeds the theoretical prediction in the largest pore size, underscoring the necessity for theories that consider multiscale confinement effects. This study presents a novel nanofluidic approach to elucidate nanoscale fluid miscible behaviors and the impact of pore structures, providing an important strategy for quantifying fluid miscibility in complex porous media.