Wettability Of Water On Carbon Nanomaterials

Wettability is one of the important fundamental properties of the solid surface. Materials with different wetting properties have wide range of potential applications. The wettability of solid surface can be effectively changed by tuning the surface microstructure. Besides, it can be also prominently affected by various external conditions, such as external electric fields. People usually come into contact with the phenomena of droplet coalescence, such as the cloud formation in the nature, the emulsification and the 3D pringting technology. The wettability and surface structure play a crucial role on the morphological evolution of droplet and the water film coalescence. Therefore, it is necessary to study the influence of surface structure and external electric fields on wettability which is of great significance on tuning the wetting properties and developing materials with dewetting properties.In this paper, we carried out classical molecular dynamics simulation to figure out the evolution of water on graphene and carbon nanotubes and the effects of electric fields on wettability of graphene. We revealed the inner connection between interfacial properties, anisotropic surface topography and water coalescence, and found the method to change wettability of water on graphene. The main results are as follows:(1) The coalescence behaviors of identical water films on graphene (G), horizontally or vertically arranged carbon nanotube array (HCNTA and VCNTA) substrates have been studied. Our results show that the height of liquid bridge increases linearly with simulation time. And the liquid bridge width can be roughly described by the scaling law, ho～t1/2, when water films are located on G and HCNTA substrate. Two relatively intense density peaks can be observed on the mass-density distribution curves for three systems, which indicate the ordered water layer. And the lower diffusion speed of water molecules in the direction perpendicular to the surface arising from the relatively ordered water layer is responsible for the difference of coalescence rate.(2) We studied the effect of anisotropic surface topography on water film coalescence. On different rough surface, the height of the liquid bridge still displays a linear increasing tendency. Moreover, on the rough surface with the larger water contact angle (WCA), the growth rate of liquid bridge is much faster than the others. In the case of nonadjacent water films which are separated at a certain distance of 1 A, the meniscus liquid bridge occurs solely on the VCNTA, which is attributed to the spreading of water films driven by the capillary force. While the distance increases to 2 A, the water films won’t coalescence on VCNTA. Our results provide an available method to tune the coalescence of water films with the alteration of patterned surfaces, which has important implications in the design of condensation, ink-jet printing and drop manipulation on solid surface.(3) We have investigated the behavior of water droplets located on graphene in the presence of external electric fields (E-field). The influence of E-field on mass density distribution, water polarization and hydrogen bonds (H-bonds) have been researched. The MD simulation results showed that the water droplets in equilibrium system present hemispherical, conical and ordered cylindrical configuration as the E-field increases. Moreover, the dipole orientation of water molecules experiences a remarkable change from a disordered state to an ordered state due to the polarization of water molecules. The distinct two peaks in mass density and H-bond distribution profiles indicate the layered water structure in the interfacial region, which sensitively depends on the strong E-field. In addition, when the external E-field is parallel to the substrate, the E-field would increase its wettability.Our findings provide the possibility to control the water behavior and wetting properties of water on graphene or CNTs by tuning the surface structure or external E-field which is of importance for relevant industrial processes on the solid surface.