Hydrodynamics inside brush decorated nano-confinements: an all-atom molecular dynamics study

The increasing role of polymer grafting inside nano-confinements is evident in applications ranging from species separation, controlled transport, electrokinetic conversion, thermoelectric devices, and drug delivery, to enhanced cooling systems. In this work we explore the interdependencies of parameters governing the flow dynamics in these confinements at the nanoscale owing to the interplay of confinement, grafting density, and the chain lengths of the grafted polymers. We unravel these effects through pressure-driven, fully atomistic simulations and understand the role of molecular interactions between various constituents of the system and the diffusion coefficients. A key novelty of this study lies in the unique simulation methodology, where isobaric-isothermal (NPzT) simulations are employed to achieve stable, densely packed system configurations unattainable in conventional all-atomistic molecular dynamics for cases where grafting lengths are more than half of the confinement height. The system configurations thus generated can be simulated by a traditional isothermal methodology. The effects of brush length-especially when exceeding half the confinement size-and grafting density on fluid transport under external pressure fields are systematically explored through this approach. We also study the impact of the external pressure field on the orientation of the grafted polymers and the corresponding effect on the enhancement factor to devise a universal parameter called the effective confinement size ratio that can be utilized to engineer the confinement parameters for various applications. The parameter can be used as an upscaled manifestation of effective flow rate giving a generalized correlation for varying grafting characteristics and applied flow fields in nano-confinements.