Interface-induced dispersion in the unsaturated porous media: A pore-scale perspective

Although the fluid invasion process impacts the transport and dispersion of chemicals and contaminants in the resident phase of natural, manufactured, and biological unsaturated porous media, these effects remain poorly understood. In this study, we investigate the role of hydrodynamic dispersion in different invasion patterns within the resident phase. While nonidealities in pore structures contribute to mechanical dispersion, our findings demonstrate that the interface front's morphology and the invading phase's distribution significantly influence mechanical dispersion within the resident phase. Our results show that the complex distribution of the invading phase under unstable displacement greatly affects mechanical dispersion measures, such as velocity distribution, variance, and mean square displacement obtained through particle tracking. The interface also alters hydrodynamic dispersion measures, such as the local Peclet number, and generates extensive diffusion-dominated regions across the domain. This ultimately leads to solute trapping between pores that cannot reach the outlet. However, under stable invasion, mechanical and hydrodynamic dispersion do not deviate significantly from the reference saturated condition. In this scenario, dispersion anomalies occur within a short distance ahead of the moving interface, beyond which variations become similar to the reference media.