Gaining insights into capillary barrier effects through microfluidic experiments incorporating gravity

This study performed microfluidic experiments of water displacing air at a low injection rate to provide deeper insights into capillary barrier effects (CBEs), which are encountered in numerous geotechnical and geoenvironmental applications. Four microfluidic chips with varying pore networks, including one single-layer and three double-layer configurations, were used to examine how pore characteristics impact CBEs. Tilt angles were systematically applied to the chips to explore the influence of gravity on CBEs. The results revealed that the introduction of gravity induced directional water invasion, producing downward preferential flows. CBEs were clearly observed when water moved from fine to coarse layers, causing the water motion direction to shift from downward to lateral or backward, and resulting in perched water above the fine–coarse interfaces. Notably, the pause duration of the advancing water front proved to be a more reliable indicator of CBEs’ effectiveness at the microscale than the water storage capacity of fine layers. The formation of CBEs required that the maximum driving pressure on the water–air interface be provided by fine layers after the water front reached the fine–coarse interfaces. A smaller gravitational force and larger pore sizes in coarse layers reduced the driving pressure provided by coarse layers, thereby enhancing the effectiveness of CBEs.