Hydrodynamic slip length as a surface property

Abstract

Equilibrium and nonequilibrium molecular dynamics simulations were conducted in order to evaluate the hypothesis that the hydrodynamic slip length is a surface property. The system under investigation was water confined between two graphite layers to form nanochannels of different sizes (3-8 nm). The water-carbon interaction potential was calibrated by matching wettability experiments of graphitic-carbon surfaces free of airborne hydrocarbon contamination. Three equilibrium theories were used to calculate the hydrodynamic slip length. It was found that one of the recently reported equilibrium theories for the calculation of the slip length featured confinement effects, while the others resulted in calculations significantly hindered by the large margin of error observed between independent simulations. The hydrodynamic slip length was found to be channel-size independent using equilibrium calculations, i.e., suggesting a consistency with the definition of a surface property, for 5-nm channels and larger. The analysis of the individual trajectories of liquid particles revealed that the reason for observing confinement effects in 3-nm nanochannels is the high mobility of the bulk particles. Nonequilibrium calculations were not consistently affected by size but by noisiness in the smallest systems.

Nanoscale hydrodynamics near solids

“…Another recent approach has considered a generalized Langevin equation for the velocity of a single atom of the fluid near a wall. 31 Recent work by Ramos-Alvarado et al 32 compares the above different approaches and concludes that simulations are very sensitive to post-processing issues, leaving the story somewhat inconclusive.…”

“…Our theoretical approach is not restricted to parallel flow (i.e., fluid slabs) as is typically considered in simulation work. 13,26,29,30,32 Most derivations of Green-Kubo expressions for friction are based on linear response theory where the Hamiltonian is slightly perturbed by an external forcing. 13,31 We follow here a different approach in obtaining directly the full hydrodynamic equations from the microscopic dynamics.…”

Large effect of lateral box size in molecular dynamics simulations of liquid-solid friction

“…In slab simulations, two types of finite size effects should be considered. On the one hand, the effect of the confinement height has been explored in previous work [27][28][29]36,37,[43][44][45]. On the other hand, the effect of the lateral size of the simulation box has been little explored.…”

Statistical Mechanical Expressions of Slip Length

“…We now develop the linear response theory that describes the macroscopic behavior at the time τ = τ meso . We prepare an initial state chosen according to (41), and consider the time evolution during the time interval [0, τ meso ]. In this interval, the macroscopic behavior of the fluid is determined by the velocity fields u x (z) given as the initial condition.…”