Microbial nitrogen bubble formation in porous media

Microbially induced nitrogen (N₂) gas bubbles can desaturate subsurface areas and thus have been considered as an alternative ground improvement technique for mitigating soil liquefaction potential caused by earthquakes. However, the detailed mechanisms of subsurface N₂ bubbles are not well understood and remain a subject of ongoing research. In this study, a transparent microfluidic device was utilized to mimic biological N₂ gas bubble formation by nitrate-reducing bacteria and to visually characterize the entire process. During N₂ gas formation, a limited number of bubble nucleation sites were identified, which gradually expanded upward through the preferential pore channels. N₂ gas bubbles tended to create interconnected gas pockets rather than existing as evenly distributed small gas cavities. The degree of water saturation gradually reduced over a week as the bubbles were produced. The gas ganglia repeatedly grew until they reached the top boundary, which triggered a drastic expulsion of bubbles by ebullition. Despite fluctuations in saturation level, the residual saturation was maintained at around 73 %. Comparative experimental case studies of CO₂ gas bubble formation were conducted to identify contrasting gas formation mechanisms. CO₂ gas bubbles were generated via the abiotic decompression of a supersaturated CO₂ solution under two distinct rates of pressure reduction. Rapid CO₂ bubble formation led to uniform nucleation and 41 % residual saturation, while slower formation yielded 35 % due to stable liquid displacement by the gas front. This study highlights the potential of the microfluidic device as an experimental tool for visualizing subsurface gas formation mechanisms. The insights gained could further enhance and optimize geotechnical applications involving gas formation in highly saturated soils.