Molecular Dynamic Simulation Of Wetting Behavior On Nanofiber-structured Surface And POSS-based Material

Superhydrophobic material is one of the hotpoints in scientific research. It can be utilized as self-cleaning materials and surface coat, and applied in the fields of textile, electronics, medicine, aerospace and so on. For superhydrophobicity, it's related to both the micro structure and chemical properties of the material. In this work, molecular dynamics simulation has been used to investigate wetting behavior from the above two aspects. The main contents are as follows:For single atom system of copper and mercury, molecular dynamic simulation is carried out on the validation of the simulation method and on the investigation of the influence of temperature, lateral dimension and radius of liquid droplet on the surface wetting behaviors. It is found that as varying the interactive parameter, the value and trend of contact angle are close to those from the reference. It suggests that it's reasonable to simulate the wettability with the proposed approach in this work. And the effect of lateral dimension can be ignored due to the application of periodic boundary condition. Microscopically, while enlarging the radius of liquid droplet, it improves the hydrophobicity of the material as a result of line tension. Accordingly, it is possible to predict contact angle at macroscopic experiments, and reduce the gap between the simulated and the experimental contact angles. In addition, decreasing the temperature can increase contact angle, and enhance the hydrophobicity of the surface.Based on the simulation of ideal surface, nano fiber-structured surface was built up to study the effect of the micro structure on the wettability. With the increase of the height of nano fibers, simulated contact angle increases at first, and then tends towards stability. While increasing the pitch between nanofibers, contact angle decreases following the increase at first. They are in good agreement with Wenzel's and Cassie's equations. When investigating the influence of the diameter of nanofibers, critical equilibrated contact angle can well predict the transition between the homogeneous and the heterogeneous wetting states, which is due to the existence of free-energy barrier. It should also be noted that when initial height between the droplet and nanofiber-structured surface is approximate to colloid diameter, it can affect both wetting state and contact angle.Finally, the wettability of polyhedral oligomeric silsesquiones (POSS) is investigated by molecular dynamics simulation and compared with the experiment, in which the sample is synthesized with the method of direct hydrolytic condensation. Force fields of COMPASS and SPC are assigned to POSS and water, respectively. It is shown that the substrate of H-POSS is crystallizable to some extent and performs hydrophobicity. The good consistence between simulated and experimental contact angles can validate that molecular dynamic simulation can be utilized to model the wettability of H-POSS. It is also found that with the increase of temperature, hydrogen bond in water is reduced. Therefore, the hydrophocicity of H-POSS is weakened at high temperature. For POSS with one or eight substitutions, the simulated contact angle is increased with the enlargement of the alkyl chain. It suggests the hydrophobicity is strengthened. As for POSS-based materials, the introduction of POSS cage improves the hydrophobicity of the material compared with polyethylene.