The Impact of Mixed Wettability on Pore-Scale Fluid Displacement Dynamics in Microfluidic Models

Abstract This study explores the role of mixed wettability in influencing fluid displacement behaviors at the pore scale, which is a critical yet underexplored aspect of subsurface fluid flow. By replicating natural porous media conditions in microfluidic devices, we investigated how wettability heterogeneity influences injection time, displacement patterns, dynamic pressure, and sweep efficiency. These findings provide insights relevant to applications such as enhanced oil recovery, and CO2 sequestration, among others. A series of microfluidic devices featuring rock-like structures with controlled wettability variations were fabricated using photolithography and molecular vapor deposition (MVD) of perfluorodecyltrichlorosilane (FDTS). Predominantly hydrophobic surfaces were selectively modified in narrow pore throats based on pore size distribution, effectively mimicking the mixed wettability characteristics commonly observed in natural rock formations. Experimental setups included water injection into air-saturated micromodels under capillary-dominated conditions, with real-time high-resolution imaging capturing pore-scale displacement patterns. A range of wettability conditions including fully hydrophobic, fully hydrophilic, and mixed wet, was systematically tested to assess the influence of wettability on fluid dynamics. The findings reveal that wettability significantly impacts fluid displacement dynamics, with distinct behaviors observed for different wettability conditions. Dynamic pressures were inversely proportional to the fraction of hydrophilic zones, reflecting the influence of wetting pore throats on invasion pressure and pressure curve shifts. Saturation trends and pore-filling sequences varied across wettability conditions, highlighting the critical role of localized wettability changes in redirecting fluid flow. Mixed wettability introduced a balance between capillary and viscous forces, creating unique displacement behaviors not observed in uniform-wet systems. These findings challenge conventional assumptions by demonstrating that mixed-wet systems exhibit unique and complex dynamics rather than serving as intermediates between uniform extremes. This study employs high-resolution microfluidic models with pore-size-correlated mixed wettability distributions, establishing a robust framework for analyzing fluid dynamics in heterogeneous porous media. The results advance our understanding of multiphase flow in mixed-wet systems, offering practical insights for optimizing fluid management in subsurface energy applications.