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

Koshun Ogawa,Takeshi Omori,Hiroki Kusudo,Laurent Joly,Haruki Oga,Yasutaka Yamaguchi,Samy Merabia

doi:10.1103/physreve.100.023101

Abstract

Molecular dynamics simulations are a powerful tool to characterize liquid-solid friction. A slab configuration with periodic boundary conditions in the lateral dimensions is commonly used, where the measured friction coefficient could be affected by the finite lateral size of the simulation box. Here we show that for a very wetting liquid close to its melting temperature, strong finite size effects can persist up to large box sizes along the flow direction, typically ∼30 particle diameters. We relate the observed decrease of friction in small boxes to changes in the structure of the first adsorbed layer, which becomes less commensurable with the wall structure. Although these effects disappear for lower wetting cases or at higher temperatures, we suggest that the possible effect of the finite lateral box size on the friction coefficient should not be automatically set aside when exploring unknown systems.

Highlights

Nanofluidic systems [1,2] show great promise for applications such as blue energy [3,4,5] and water desalination [6,7,8]
We relate the observed decrease of friction in small boxes to changes in the structure of the first adsorbed layer, which becomes less commensurable with the wall structure. These effects disappear for lower wetting cases or at higher temperatures, we suggest that the possible effect of the finite lateral box size on the friction coefficient should not be automatically set aside when exploring unknown systems
To measure liquid-solid friction in molecular dynamics simulations, a slab configuration is commonly used, where a liquid is confined between parallel walls

Summary

INTRODUCTION

Nanofluidic systems [1,2] show great promise for applications such as blue energy [3,4,5] and water desalination [6,7,8]. When liquids are confined at nanometric scales, surfaces play an increasingly important role over bulk liquid properties, and it is critical to better understand and control the behavior of liquid-solid interfaces. Both experiments and molecular simulations have shown that liquids can slip on some surfaces at the nanoscale [9], which can enhance the performance of nanofluidic systems [10,11,12,13,14]. Previous MD work on liquid-solid friction cited above typically used lateral box sizes ranging. In this work we would like to assess the validity of such box sizes by examining finite size effects on liquid-solid friction. We will show that significant finite size effects can persist up to large box sizes for very wetting liquids close to the melting temperature, and we will show the atomic origin of these effects

SYSTEMS AND METHODS

RESULTS AND DISCUSSION

CONCLUSION