Comments on `Irreversibility in Response to Forces Acting on Graphene Sheets'

In the letter "Phys. Rev. Lett 104, 196804 (2010)" the compression-relaxation mechanism (CRM) for a graphene sheet (GS) was reported to be irreversible and resulted in static ripples on GS, such that for T <Tc the free-energy of the rippled GS is smaller than that of roughened GS. We will point out several technical difficulties, such as the use of the relation Delta A= for the free energy calculations and the definition of the rough state, with their simulations. We show that (at T <Tc) their introduced rough state suffer boundary stress, thus is a rippled state and their obtained inequality A_{ripple}<A_{rough} is no longer valid. Therefore the introduced mechanism is reversible. Furthermore, from theoretical point of view for an infinitely slow rate of CRM, the relation Delta A= (in common non-equilibrium simulations) is allowed. In this case authors of Ref. [1] must report the used infinitely small rate and justify how it is in practice valid. We show Delta A for the system that was used in [1]

Commensurability Effects in Viscosity of Nanoconfined Water

The rate of water flow through hydrophobic nanocapillaries is greatly enhanced as compared to that expected from macroscopic hydrodynamics. This phenomenon is usually described in terms of a relatively large slip length, which is in turn defined by such microscopic properties as the friction between water and capillary surfaces, and the viscosity of water. We show that the viscosity of water and, therefore, its flow rate are profoundly affected by the layered structure of confined water if the capillary size becomes less than 2 nm. To this end we study the structure and dynamics of water confined between two parallel graphene layers using equilibrium molecular dynamics simulations. We find that the shear viscosity is not only greatly enhanced for subnanometer capillaries, but also exhibits large oscillations that originate from commensurability between the capillary size and the size of water molecules. Such oscillating behavior of viscosity and, consequently, the slip length should be taken into account in designing and studying graphene-based and similar membranes for desalination and filtration.Comment: 21 pages, 5 figures, ACS Nano, ASAP (2016