A preliminary exploration of the micro-scale behaviour of capillary barrier effect using microfluidics

The capillary barrier effect (CBE) is employed in a large number of geotechnical applications to decrease deep percolation or increase slope stability. However, the micro-scale behaviour of CBE is rarely investigated, and thus hampers the scientific design of capillary barrier systems. This study uses microfluidics to explore the micro-scale behaviour of CBE. Capillarity-driven water flow processes from fine to coarse porous media with different pore topologies and sizes were performed and analysed. The experimental results demonstrate that the basic physics of CBE is the preferential water movement into the fine porous media due to the larger capillarity. The effects of CBE on water flow processes can be identified as delaying the occurrence of breakthrough into the coarse porous media and increasing the water storage of the fine porous media. The CBE can impede the increase of the normalised length and decrease the normalised width of the water front, suggesting that the two normalised parameters are potential indicators to assess the performance of CBE at the micro scale. CBE can be formed in square and honeycomb networks with the ratio of coarse to fine pore throat width larger than 2·0 when gravity is neglected, and its performance can be affected by pore topology and size.