Dispersion_Wettability_dataset

Solute spreading and mixing critically influence reactive transport predictions and remediation efficacy in unsaturated aquifers. However, the role of wettability in controlling solute mixing within the aqueous phase of immiscible two-phase flow remains insufficiently understood. In this study, we systematically modified the surface wettability of silica sand through physicochemical treatment to establish four distinct wetting conditions: strongly water-wet (SWW), water-wet (WW), neutral-wet (NW), and oil-wet (OW). Using synchrotron-based high-resolution X-ray computed tomography, we directly visualized and quantified the four-dimensional evolution of NaI tracer transport in oil–water fractional flow systems. Increasing oil-wetness enhanced aqueous-phase flow heterogeneity and produced a rightward shift in the relative permeability crossover point. Dispersion and mixing are suppressed in SWW and NW systems but enhanced in WW and OW systems. Analysis of the scalar dissipation rate revealed three characteristic temporal patterns—standard decreasing (╮), S-shaped (∽), and parabolic (⁔) —each reflecting distinct interfacial flow dynamics. In particular, NW systems exhibite suppressed mixing due to constrained interfacial area, while WW and OW conditions enhance mixing through preferential flow and droplet-induced interfacial renewal. These observations are identified by a quantitative correlation between solute plume curvature and mixing intensity, where curvature-driven stretching accelerates early-stage mixing, followed by a transition to diffusion-dominated regimes governed by local concentration gradients. These results provide new mechanistic insight into wettability-controlled solute mixing and dispersion within the aqueous phase of immiscible two-phase flow and highlight implications for predicting non-Fickian transport in subsurface systems.