Nanocapillaries are essential for a wide range of applications, from enhanced oil recovery to the study of fluid dynamics at the nanoscale. This study examines the dynamics of miscible flow in planar nanochannels under pressure-driven Poiseuille flow conditions. We conducted extensive molecular dynamics (MD) simulations to investigate the microscopic mechanisms governing solute displacement and dispersion within nanoscale confinement. As a representative case, we focused on ${\mathrm{CO}}_2$ dispersion in oil (n-decane) across various channel materials, estimating average velocity, slip lengths, and diffusion coefficients. Our findings reveal significant differences in slip lengths between various channel materials, with graphene exhibiting a slip length over an order of magnitude higher than others. We estimated the slip length of oil on graphene to be 4.5 nm, an order of magnitude smaller than that of water on graphene, whereas for ${\mathrm{MoS}}_2$ and silica, it was smaller than 1 nm. From MD simulations, we found that the diffusion coefficient increases nonlinearly with channel height and applied pressure, ranging from $\left[4\text{–}20\right]\times {10}^{-9}{\text{m}}^2/\text{s}$ for the studied conditions. Taylor analysis was used to qualitatively interpret these findings. This study enhances the understanding of the slippage dynamics and diffusion processes in nanoconfined space, particularly focusing on CO-oil systems. These findings provide insights that may be applicable to other solute-solvent systems, offering a new approach for extending theories to different two-phase flow and multiphase flow systems.

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• Doubly diffusive convection
• Low-dimensional models
• Multiphase flows
• Nanofluidics