Research On Molecular Dynamics Simulation Of Surface Microstructure Control Of Wettability

Wettability is one of the important characteristics of solid surfaces,which can affect many physical and chemical processes,and has important application prospects in the fields of materials,energy,and the environment.How to adjust the wettability of materials by modifying the micro-nano structure of the surface to meet people’s actual needs for materials（such as self-cleaning,high-efficiency separation,and nano-superwetting properties,etc.）is still the research of significant practical value.It has important guiding significance for the application and development of materials.At the same time,fully understanding the basic principles of infiltration at the microscopic scale will help to develop infiltration theory,explore interface science,and lay a theoretical foundation for regulating macroscopic infiltration and other behaviors.With the development of computers,theoretical simulation methods can easily study the phenomena on the microscopic interface from the molecular scale.Therefore,the study of the wetting properties of the solid-liquid interface at the nanoscale and the exploration of the mechanism of related phenomena through the molecular dynamics simulation method are of great significance for the development of interfacial science and the preparation of multifunctional materials.The main work is as follows:（1）The size effect of the nano-droplets contact angle.The contact angle（θ）is the most commonly used method to measure interfacial wettability in both experimental and theoretical studies and the relationship between cosθand 1/r has been controversial due to the existence of line tension.This work systematically investigates the contact angle of droplets with different sizes on hydrophilic and hydrophobic surface using molecular dynamics simulation.The results show that there are two types of modified Young’s equation relationship between cosθand 1/r from hydrophilic to hydrophobic surfaces:first-order correction（weakly hydrophobic or hydrophobic surfaces）and second-order correction（hydrophilic surfaces）.In the simulated system,when the macroscopic contact angle is～82.11°,the line tension under this system has the least effect on the contact angle,which is mainly affected by the surface tension.As the increase of hydrophobicity,the effect of line tension on the contact angle gradually increased.In addition,by changing the position of the solid-liquid interfaceδh,it is found that for a system with a macroscopic contact angle of～138.25°（whenδh=σ_O）,the relationship between cosθand1/r can be directly deduced from the relationship between surface tension and radius.（2）Research on tunable wettability of titanium dioxide surface.Based on a series of discussions and controversies caused by the role of-OH in the mechanism of light-induced hydrophilic effect on the surface of titanium dioxide and the existence of organic acids,this study quantitatively analyzed the influence of-OH and carboxylic acids on interface wettability used molecular dynamics simulation.Firstly,first-principles calculations demonstrate the adsorption stability of various molecules on the Ti O₂surface.The research results of wettability show that the more-OH content after water molecule dissociated-adsorption,the stronger hydrophilicity of the surface,which indirectly proves the conclusion that-OH can promote surface hydrophilicity in the photo-induced hydrophilic effect.The dissociative adsorption of formic acid and acetic acid in carboxylic acids also makes the surface more hydrophilic,with formic acids being more hydrophilic than acetic acids.However,the dissociated of propionic acid～caproic acid all show that the weaker hydrophilicity of the surface,until the surface is completely dissociated by caproic acids,the contact angle appears to be～90°.This study reveals why the adsorption of short alkyl chain carboxylic acids makes the surface more hydrophilic from molecular point,and it also provides another mechanism explanation for Ti O₂light-induced hydrophilicity and provides important theoretical support for the regulation of interface wettability.（3）Study on the Mechanism of Self-cleaning effect on Titanium Dioxide Surface.According to the conclusion that carboxylic acids in the air can be selectively adsorbed on the Ti O₂surface detected in the experiment,the effect of HCOOH～C₁₀H₂₁COOH on the self-cleaning effect of the Ti O₂surface is systematically studied through molecular dynamics simulations in this work.The results show that the dissociated-adsorption of short alkyl chain carboxylic acids（formic acid/acetic acid）promotes the excellent self-cleaning ability of the surface,and the removal ratio of surface pollutants can reach80%to 90%.It is significantly higher than that of all other systems（including the homogeneous Ti O₂surface without any adsorption）,and it can be found that the acetic acid is better than the formic acid by comparing the difference in effect between two short alkyl chain carboxylic acids.With the increase of the alkyl chain,the self-cleaning ability gradually weakened until it disappeared.This study is the first time to discuss the influence of carboxylic acids on the self-cleaning effect of Ti O₂surface from a microscopic point of view in detail,especially the microscopic kinetic process promoted by formic acids and acetic acids,which provides a certain theoretical guidance for the development of self-cleaning materials.（4）Mechanism Study on the Difference of Redispersion Ability for Mo S₂Stacked Films.In view of the fact that it is difficult to explain the difference of redispersibility ability of the stacked Mo S₂nanosheets from the molecular level in experiments.In this work,the microscopic process of the aggregation and redispersion of Mo S₂nanosheets of two phases in aqueous solution is studied by the steered molecular dynamics simulations.The results show that Mo S₂nanosheets in two phase states（1T and 2H）will exhibit different interlayer spacings（1T phase:～12.3（?）,2H phase:～6.4（?））during the aggregation process,which is consistent with the conclusions obtained in the experiment.The surface properties of the material and the aggregation mode jointly affect its redispersion ability after aggregations.The redispersion energy barrier of the hydrophobic 2H phase is greater than that of the hydrophilic 1T phase for the same parallel aggregated Mo S₂nanosheets.The change of the aggregation mode makes the energy barrier that needs to be overcome in the redispersion process of Mo S₂nanosheets gradually decrease with the increase of the tilt angle,and the redispersion behavior is more likely to occur.This study provides strong theoretical evidence for explaining the reasons for the differences in redispersibility and promotes the green development of materials.In summary,this paper starts from several key issues related to the wettability of solid-liquid interface,combining first-principles calculations and molecular dynamics simulations,the relationship between the contact angle and size of nano-droplets,the quantitative issues in the regulation of interfacial wetting properties,and the mechanism of related important phenomena are systematically studied.This series of research results has enriched the understanding of the relationship between the droplet contact angle and the size,and promoted the understanding of the molecular mechanism of interface wetting behavior,and provides a new strategy for the regulation of material wettability,and provides corresponding theoretical guidance for the utilization of diversified materials,which is of great significance for controlling material properties and designing new materials.