Wetting and Drying Transitions of Water Nanodroplets on Suspended Graphene Bilayers

چکیده مقاله

Room-temperature wettability of a single graphene sheet has long been a controversial subject. To gain a better understanding of the problem, temperature dependence of the contact angle (CA) of water nanodroplets with the surface of suspended graphene bilayers was investigated by extensive molecular dynamics (MD) simulations. We also estimated the macroscopic CA ΘM and its dependence on temperature. Four popular Lennard-Jones (LJ) energy parameters ϵCO for water oxygen−carbon pair interactions, corresponding to ΘM = 23 ± 2, 64 ± 4, 87 ± 3, and 127 ± 2° at 300 K, were utilized in the MD simulations in order to cover a broad range of possible roomtemperature wettability. The MD simulations indicate that for the first three cases with ΘM < 90°, a wetting transition occurs at both nano- and macroscales at a characteristic temperature Tw below the water critical temperature Tc. The general trends of the dependence of ΘM on the temperature T and the estimate of Tw for the third case (ΘM = 87 ± 3°) are in good agreement with the experimental data for water on highly oriented pyrolytic graphite surface. In the case of ΘM = 127°, water nanodroplets detach from the surface at a temperature Td < Tc, signaling a drying transition. The extrapolated values of Θ provide evidence for the drying transition at the macroscale as well. This is in contrast to three-dimensional materials for which the drying transition is believed to occur either for ϵCO = 0 or if weak water−surface interactions are truncated at short distances. The robustness of the results for both drying and wetting transitions is demonstrated by increasing the LJ cutoff radius rc up to the values that further truncation of the potential produces negligible error. In addition, the occurrence of wetting or drying transition at the macroscale was also studied by MD simulation in a slab-like system, for which the macroscopic CA was estimated by calculating it for a thin but long water droplet that is almost free of the line tension effect