Fluid interfacial nanoroughness measurement through the morphological characteristics of graphene.

The interfacial nanoroughness of liquid plays an important role in the reliability of liquid lenses, capillary waves, and mass transfer in biological cells [Grilli et al., Opt. Express 16, 8084 (2008), Wang et al., IEEE Photon. Technol. Lett. 18, 2650 (2006), and T. Fukuma et al., 92, 3603 (2007)]. However, the nanoroughness of liquid is hard to visualize or measure due to the instability and dynamics of the liquid-gas interface. In this study, we blanket a liquid water surface with monolayer graphene to project the nanoroughness of the liquid surface. Monolayer graphene can project the surface roughness because of the extremely high flexibility attributed to its one atomic thickness. The interface of graphene and water is successfully mimicked by the molecular dynamics method. The nanoroughness of graphene and water is defined based on density distribution. The correlation among the roughness of graphene and water is developed within a certain temperature range (298-390 K). The results show that the roughness of water surface is successfully transferred to graphene surface. Surface tension is also calculated with a simple water slab. The rise of temperature increased the roughness and decreased the surface tension. Finally, the relationship between graphene roughness and surface tension is fitted with a second-order polynomial equation.