We study the preference of two-phase displacements systematically by theoretical derivations and numerical simulations via a non-uniform pore doublet model. All the most important impact factors, including viscosity ratio, capillary number, wetting conditions and boundary conditions, have been considered, and finally a complete phase diagram for preferential flow has been obtained. The simple treatment for the dual-permeability media has been validated, and further, with a few necessary corrections the phase diagram is applicable for disordered permeable media in general. These results help us to understand the occurrence and manipulation of preferential flow in heterogeneous permeable media.

We report non-monotonic wettability effects on displacement efficiency in heterogeneous porous structures at the post-breakthrough stage, in contrast to the monotonic ones in homogeneous porous structures. Experiments on designed microfluidic chips show that there exists a critical wettability to attain the highest efficiency of displacement in the porous matrix structure combined with a preferential flow pathway, while a stronger wettability of the displacing fluid leads to a higher displacement efficiency on the same matrix structure only. The porous structure with or without a preferential flow pathway results in totally different topological characteristics of phase distribution during displacement. Pore-scale mechanisms are identified to elucidate the formation of this non-monotonic wettability rule: cooperative pore filling under weakly water-wet conditions yields the best displacement; corner flow under strongly water-wet conditions and Haines events under strongly oil-wet conditions decrease the displacement efficiency. The pore-scale findings may provide unique insights into the joint effects of both wettability and flow heterogeneity on fluid displacement in porous media.

We investigate the impact of wettability distribution, pore size distribution and pore geometry on the statistical behaviour of trapping in pore-throat networks during capillary displacement. Through theoretical analyses and numerical simulations, we propose and prove that the trapping patterns, defined as the percentage and distribution of trapped elements, are determined by four dimensionless control parameters. The range of all possible trapping patterns and how the patterns are dependent on the four parameters are obtained. The results help us to understand the impact of wettability and structure on trapping behaviour in disordered media.

Hierarchical and hollow SnS2 nanostructures as precursors were fabricated via a surfactant-assisted assembly process using sodium dodecyl sulfate as soft templates. The as-prepared SnS2 nanostructures were further oxidized to form porous SnO2 conversion for investigating their gas-sensing properties in drug-precursor detection. On the basis of a series of time- and ratio-dependent reactions, a formation mechanism of the special nanostructures and factors influencing morphology and structure were determined. Gas-sensing measurements revealed that the porous and hierarchical SnO2 hollow nanostructures were sensitive to drug precursors, indicating promising applications in environmental monitoring and public safety investigation. In addition, we found that the assembled SnO2 nanomaterials possessed significantly enhanced gas-sensing properties compared with unassembled SnO2 with a solid interior.