Direct nonequilibrium molecular dynamics simulation of diffusio-osmotic flow in nanopores.

HYPOTHESIS: Surface-driven flow, associated with solute concentration gradient in the presence of an interface, is ubiquitous in nature and occurs in various natural systems, including the human body, as well as in novel micro/nanofluidic systems. Despite its importance, the microscopic mechanism underlying diffusio-osmotic flow is not completely understood.

SIMULATIONS: We perform direct nonequilibrium molecular dynamics simulations of a binary fluid mixture confined in a nanopore, connecting two bulk reservoirs with an imposed solute concentration gradient. Unlike previous simulation studies that mimic chemical potential gradients through externally applied forces, the present large-scale simulations directly generate diffusio-osmotic flow and permit statistically reliable determination of flow rates over sufficiently long time scales.

FINDINGS: The simulations reveal that the direction and magnitude of diffusio-osmotic flow are governed by the relative strengths of solute-wall and solvent-wall interactions. When solute-wall interactions are stronger (weaker) than solvent-wall interactions, the interfacial concentration gradient is amplified (attenuated), leading to reversal of the interfacial pressure gradient and consequently of the diffusio-osmotic flow. Calculations of the local pressure tensor demonstrate that a tangential pressure gradient develops within the interfacial layer, while vanishing in the bulk fluid, providing direct molecular evidence that diffusio-osmosis is an interfacially driven phenomenon. Incorporation of this pressure gradient into the Stokes equation yields velocity profiles in reasonable agreement with the molecular dynamics results and comparable to predictions based on conventional continuum formulations. These findings establish a molecular mechanical interpretation of diffusio-osmosis and provide a unified framework connecting osmosis, diffusio-osmosis, and diffusio-phoresis.