Wetting dynamics of a water nanodrop on graphene

Abstract

Water-graphene wetting interactions are central to several applications such as desalination, water filtration, electricity generation, biochemical sensing, fabrication of fuel cells, and many more. While substantial attention has been devoted to probe the wetting statics of a water drop on graphene, unraveling the possible wetting translucency nature of graphene, very little research has been done on the dynamics of wetting of water drops on graphene-coated solids or free-standing graphene layers. In this paper, we employ molecular dynamics (MD) simulations to study the contact and the spreading of a water nanodrop, quantifying its wetting dynamics, on supported and free-standing graphene. We demonstrate that nanoscale water drops establish contact with graphene by forming patches on graphene, and this patch formation is hastened for graphene layer(s) supported on hydrophilic solids. More importantly, our results demonstrate that the nanodrop spreading dynamics, regardless of the number of graphene layers or the nature of the underlying solid, obey the half-power law, i.e., r∼t(1/2) (where r is the wetting contact radius and t is the spreading time) for the entire timespan of spreading except towards the very end of the spreading lifetime when the spreading stops. Such a spreading behavior is exactly analogous to the spreading dynamics of nanodroplets for standard solids - this is in sharp contrast to the wetting statics of graphene where the wetting translucency effect makes graphene different from other standard solids.

Roughness-Induced Chemical Heterogeneity Leads to Large Hydrophobicity in Wetting-Translucent Nanostructures

“…The wetting translucency of graphene signifies that the wetting characteristic of a particular graphene layer depends on the wettability (with respect to the original wettability of this graphene layer) of the underlying substrate. [32][33][34][35][36] This is explained by the Molecular Dynamics (MD) simulation results of our previous paper 34 and also shown in Figs. 1(a-c).…”

“…34 Here, we provide a summary of the main simulation details and describe any differences from earlier simulations. All simulations are carried out in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) 42 software package.…”

“…The capillary and the viscous time scales for the drop are τ c = ρR 3 / γ ≈ 40 picoseconds and τ v = ρR 2 / η ≈ 25 picoseconds (using radius R=5 nm, density ρ=1000 kg/m 3 , surface tension γ=0.07 N/m, and dynamic viscosity η=0.001 Pa.s) -therefore, we run our simulations for time (1000 ps) that is much larger than these two timescales that typically dictate the drop spreading dynamics. 34,54 Further, previous similar studies on drop dynamics on graphene layers have considered equilibrium at such total simulation times (1000 ps). 27,28 Consequently, we can safely assume that there will be no further change in the drop equilibrium behavior allowing us to infer that the drop equilibrates with such pinned contact lines, enforcing a large contact angle of ~ 130 0 , with the drop being in the Wenzel state.…”

“…1(a-c). For example, for a monolayer of graphene supported on bare gold, the water drop contact angle is around 75 0 , which is between the water drop contact angles on unsupported graphene (~90 0 ) 34 and bare gold (~29 0 ). 34 Of course, this water-drop contact angle on bare gold is possibly due to the adsorption of some contaminants, given that one would typically expect a zero water-drop contact angle on an absolutely pure metal surface.…”

Structure and Transport Properties of Water and Hydrated Ions in Nano‐Confined Channels

“…[1,2] Many novel applications such as desalination, energy conversion, and molecular sensing [3][4][5][6][7] have been realized by nanoporous devices. [1,2] Many novel applications such as desalination, energy conversion, and molecular sensing [3][4][5][6][7] have been realized by nanoporous devices.…”