The relationship between induced fluid structure and boundary slip in nanoscale polymer films

TL;DR

This study uses molecular dynamics simulations to explore how nanoscale polymer films slip at interfaces, revealing a transition in friction behavior related to fluid structure and shear rate, with implications for understanding nanoscale flow.

Contribution

It uncovers the relationship between induced fluid structure and boundary slip, demonstrating a universal friction-structure relationship across various interface parameters.

Findings

Slip length exhibits a local minimum at low shear rates and increases at higher shear rates.

Friction coefficient transitions from constant to power-law decay with slip velocity.

Universal relationship between friction coefficient and fluid structure is established.

Abstract

The molecular mechanism of slip at the interface between polymer melts and weakly attractive smooth surfaces is investigated using molecular dynamics simulations. In agreement with our previous studies on slip flow of shear-thinning fluids, it is shown that the slip length passes through a local minimum at low shear rates and then increases rapidly at higher shear rates. We found that at sufficiently high shear rates, the slip flow over atomically flat crystalline surfaces is anisotropic. It is demonstrated numerically that the friction coefficient at the liquid-solid interface (the ratio of viscosity and slip length) undergoes a transition from a constant value to the power-law decay as a function of the slip velocity. The characteristic velocity of the transition correlates well with the diffusion velocity of fluid monomers in the first fluid layer near the solid wall at equilibrium. We also show that in the linear regime, the friction coefficient is well described by a function of a single variable, which is a product of the magnitude of surface-induced peak in the structure factor and the contact density of the adjacent fluid layer. The universal relationship between the friction coefficient and induced fluid structure holds for a number of material parameters of the interface: fluid density, chain length, wall-fluid interaction energy, wall density, lattice type and orientation, thermal or solid walls.