Distinct Chemistries Explain Decoupling of Slip and Wettability in Atomically Smooth Aqueous Interfaces

Abstract

Despite essentially identical crystallography and equilibrium structuring of water, nanoscopic channels composed of hexagonal boron nitride and graphite exhibit an order-ofmagnitude difference in fluid slip. We investigate this difference using molecular dynamics simulations, demonstrating that its origin is in the distinct chemistries of the two materials. In particular, the presence of polar bonds in hexagonal boron nitride, absent in graphite, leads to Coulombic interactions between the polar water molecules and the wall. We demonstrate that this interaction is manifested in a large typical lateral force experienced by a layer of oriented hydrogen atoms in the vicinity of the wall, leading to the enhanced friction in hexagonal boron nitride. The fluid adhesion to the wall is dominated by dispersive forces in both materials, leading to similar wettabilities. Our results rationalize recent observations that the difference in frictional characteristics of graphite and hexagonal boron nitride cannot be explained on the basis of the minor differences in their wettabilities.

Water Flow in Single-Wall Nanotubes: Oxygen Makes It Slip, Hydrogen Makes It Stick

“…Based on our simulations, we find that irrespective of the curvature, water exhibits a ≈4–5 times larger friction coefficient on BN surfaces compared to equivalent carbon systems, reaching a maximum value of ≈4.5 × 10 4 N s m –3 and ≈17 × 10 4 N s m –3 for monolayer graphene and hBN, respectively. These friction coefficients on the curvature-free interfaces agree well with previous computational studies. ,,,− In fact, our benchmark simulations provide a reliable estimate of the absolute values which are highly scattered ranging from ≈1 to ≈10 × 10 4 N s m –3 (experiments report a friction coefficient of ≈12 × 10 4 N s m –3 on graphite) and ≈4 to ≈30 × 10 4 N s m –3 for the distinct systems. This wide spread of results can be associated with differences in the chosen force field, ,, DFT functional, , or simulation setup related to a frozen substrate, finite size errors, and thermostatting as well as confinement of water between two layers. , …”

“… 20 , 23 , 24 , 31 − 34 In fact, our benchmark simulations provide a reliable estimate of the absolute values which are highly scattered ranging from ≈1 to ≈10 × 10 4 N s m –3 (experiments 35 report a friction coefficient of ≈12 × 10 4 N s m –3 on graphite) and ≈4 to ≈30 × 10 4 N s m –3 for the distinct systems. This wide spread of results can be associated with differences in the chosen force field, 31 , 33 , 36 DFT functional, 23 , 24 or simulation setup related to a frozen substrate, 20 finite size errors, and thermostatting 37 as well as confinement of water between two layers. 24 , 32 …”

Liquid–Solid Interfaces under Dynamic Shear Flow: Recent Insights into the Interfacial Slip

“…Considering the slip length b from a geometrical viewpoint (Figure 1), it can be interpreted as a spatial point deep into the solid wall at which the fluid flow velocity extrapolates to zero. 15 In ideal cases, the slip length is proportional to the fluid viscosity:…”

“…Navier deliberated the slip characteristic of the fluid at a solid boundary and proposed a general boundary condition by defining a slip length b : where γ̇ is the local shear rate, v ∥ is the fluid velocity in the direction parallel to the solid surface, and v ∥0 is the value at the liquid–solid interface ( z = 0). Considering the slip length b from a geometrical viewpoint (Figure ), it can be interpreted as a spatial point deep into the solid wall at which the fluid flow velocity extrapolates to zero . In ideal cases, the slip length is proportional to the fluid viscosity: − where η is the fluid viscosity and k is the friction coefficient.…”

Modulating Water Slip Using Atomic-Scale Defects: Friction on Realistic Hexagonal Boron Nitride Surfaces

“…Accordingly, it is useful to understand how fast water can flow on hBN surfaces, in particular, by characterizing the slip length and friction coefficient for nanoscale fluid mechanics . To this end, molecular dynamics (MD) simulationswhether using classical potentials or using ab initio potential energy surfacesare a valuable computational tool to understand fluid slippage. , MD simulations have not only been used for understanding wetting and friction on hBN surfaces ,− but also on other 2D materials, such as graphene − and MoS 2 . − Although the presence of defects , could affect solid–liquid friction, previous studies have mostly focused on the frictional behavior of ideal, perfect hBN. , …”