Concentration driven capillary motion of liquid droplets inside chemically and topographically heterogeneous nanopores.

Abstract

We employ molecular dynamics simulations to investigate the capillary motion of liquid droplets within a nanopore featuring topographical and chemical heterogeneity along its longitudinal axis. The pore is divided into a structurally smooth region and a physically rough region composed of rectangular pillars with localized chemical variations at the pillar peaks. To modulate the wetting behavior, we introduce two-site dimeric solutes characterized by a solvent-like site and a tunable solid-like adsorption site. This architecture ensures that solutes selectively populate the solid-liquid interface, allowing us to tune the solid-liquid interfacial free energy while leaving the liquid-vapor interfacial free energy unchanged. Our results indicate that directional capillary motion can be triggered and even reversed between the smooth and rough regions through two distinct mechanisms: concentration-driven motion, where altering the bulk concentration of a single solute type shifts the droplet's thermodynamic propensity, and solute-type-driven motion, where modifying the chemical affinity of the solute at a constant concentration initiates translation. We also observed that unconfined rough surfaces allow higher solute concentrations to form a monolayer crust that physically bridges the gaps between adjacent pillars, maintaining a Cassie-Baxter-like wetting state even at low macroscopic contact angles. Conversely, nanopore confinement increases the accessible solid-liquid area, diluting solute coverage and allowing liquid to permeate the cavities. This transitions the system into a Wenzel-like state, pinning the three-phase contact line and introducing kinetic barriers. Despite these barriers, these specialized neutral solutes act as passive agents for controlling complex nanoscale fluid transport.