Surface-wettability dependent water-driving-oil two-phase flow in heterogeneous nanochannels

For the oil-water two-phase flow process in nanochannels, the oil-water wettability of the surface has a significant impact on the flow characteristics. When the wettabilities of the upper and lower surfaces are different, many interesting phenomena occur inside the channels, but no studies have been reported. This work investigates water-driving-oil two-phase flow in heterogeneous nanochannels using molecular dynamics simulations coupled with comparisons to a well-established theoretical model. As one wall evolves from hydrophilic to hydrophobic while the opposite remains hydrophilic, the flow regime sequentially evolves from symmetric-meniscus, asymmetric-meniscus, transition, to oil-film-attached flow, with variations depending on channel height and driving pressure. Flow rates exhibit nonlinear dependence on driving pressure with variations across regimes. Comparing with the theoretical model developed for predicting the time-varying imbibition length in homogeneous channels, considerable deviations are observed in asymmetric-meniscus, transition, and oil-film-attached regimes owing to additional resistances arising from heterogeneous capillary forces and oil-film adhesion.