Friction of Water on Graphene and Hexagonal Boron Nitride from Ab Initio Methods: Very Different Slippage Despite Very Similar Interface Structures

Abstract

Friction is one of the main sources of dissipation at liquid water/solid interfaces. Despite recent progress, a detailed understanding of water/solid friction in connection with the structure and energetics of the solid surface is lacking. Here we show for the first time that ab initio molecular dynamics can be used to unravel the connection between the structure of nanoscale water and friction for liquid water in contact with graphene and with hexagonal boron nitride. We find that whilst the interface presents a very similar structure between the two sheets, the friction coefficient on boron nitride is ≈ 3 times larger than that on graphene. This comes about because of the greater corrugation of the energy landscape on boron nitride arising from specific electronic structure effects. We discuss how a subtle dependence of the friction on the atomistic details of a surface, that is not related to its wetting properties, may have a significant impact on the transport of water at the nanoscale, with implications for the development of membranes for desalination and for osmotic power harvesting.Nanofluidics is an exciting field that offers alternative and sustainable solutions to problems relating to energy conversion, water filtration and desalination [1][2][3][4][5][6][7][8][9]. Miniaturization towards nanofluidic devices inevitably leads to an enhanced influence of surface and interface properties as opposed to those of the bulk. Friction is the most important interface property that limits fluid transport at the nanoscale, and its understanding is therefore crucial for the design of more efficient membranes, nanotubes and pores that exhibit low liquid/solid friction. The behavior of liquid flow at scales on the order of a few tens of nanometres departs from continuum fluid dynamics and desirable transport properties emerge at such small scales [10]. For instance, carbon nanotubes have a very high water permeability as compared to the prediction of macroscopic fluid dynamics [2]. Further, a vanishing friction has been found, giving rise to superlubric behavior of water chains inside tubes of sub-nanometre radii [11].Besides carbon, boron nitride (BN) nanostructures have recently been explored for the development of nanofluidic devices for fast water transport and efficient power generation [1,12,13]. Recent interest has been fueled by the demonstration that salinity concentration gradients across BN nanotube membranes can leed to the generation of very large electric currents [1]. It has also very recently been shown that there is a very large interlayer friction between in multiwalled BN nanotubes [14], as opposed to the superlubric behavior of the (homopolar) carbon nanotubes [15]. This suggests that the frictional properties of BN and C nanostructures might be quite different. However, to the best of our knowledge there has been no attempt to measure or compute the friction of water at the interface with BN sheets or nanotubes. Given that ab initio results have shown very similar contact angles of water drople...

Strong Coupling between Nanofluidic Transport and Interfacial Chemistry: How Defect Reactivity Controls Liquid–Solid Friction through Hydrogen Bonding

“…44 As compared to reference quantum Monte Carlo data, [45][46][47] it slightly overbinds water to graphene and BN, but the relative energy differences between different sites (relevant to friction) are well described (see the SI of Ref. 14 ). There are, of course, many types of defects that we could examine.…”

“…molecular dynamics simulations where forces are obtained from electronic structure calculations. 14,40 In particular, we have recently shown that liquid-solid friction coefficients can be computed directly from ab initio simulations, from which specific electronic structure effects on friction were revealed. 14 Here we use AIMD to investigate how defects and defect reactivity affect nanofluidic transport.…”

“…14,40 In particular, we have recently shown that liquid-solid friction coefficients can be computed directly from ab initio simulations, from which specific electronic structure effects on friction were revealed. 14 Here we use AIMD to investigate how defects and defect reactivity affect nanofluidic transport. We focus on carbon and BN surfaces, as they show great promise for nanofluidic applications, related in particular to their ultralow liquid/solid friction, which could be threatened by defects.…”

“…In previous work, 14 we performed a number of AIMD and force field MD simulations to ensure that the friction coefficients extracted from AIMD were not affected by the length of the AIMD trajectory and by the size of the system. Here, a discussion on the validity of our results for the friction coefficient in the particular case of defective sheets is reported in sections 3.2 to 3.4 of the SI.…”

Effect of critical thickness on nanoconfined water fluidity: review, communication, and inspiration

“…(1) is obtained based on bulk liquid. Besides, it is observed by many researchers that the true slip length is the function of not only the contact angle, but also many other factors (Wu et al 2017;Thompson and Robbins 1990;Joly et al 2006a, b;Suk et al 2008;Hilder et al 2009;Ho et al 2011;Tocci et al 2014;Secchi et al 2016). …”