| At present,the study of wetting properties of liquid materials at nanometer scale is very common.In the existing studies,nano droplets usually use inorganic solutions such as pure water,ethanol,and sodium chloride,but such liquids are susceptible to temperature and lead to instability of the system.Room temperature ionic liquids are a kind of green and environmentally friendly functional materials with the advantages of non-toxic,non-pollution,wide liquid path and high thermal stability,which are widely used in the study of nano-wetting characteristics.The substrate of the wetting model is mostly solid silicon,silicon dioxide,graphene,etc.,but with the in-depth study of carbon nanomaterials,it is found that the structural defects on the surface of graphene change the physical and chemical properties of graphene.Graphene oxide is the introduction of oxygen-containing functional groups after oxidation of graphene,resulting in the interface tends to be hydrophilic.Therefore,studying the wetting properties of nano-ionic liquid droplets on graphene oxide surface has potential application value.In this thesis,based on the system model of nano-ionic liquid droplets wetting on graphene oxide surface,the effects of ionic liquid droplet size and graphene oxide defect concentration on the wetting characteristics were investigated by molecular dynamics simulation.Firstly,the initial structural model of wetting of ionic liquid droplets with different radii on the surface of graphene oxide with different defect concentrations was established,and then the molecular dynamics simulation of the system was carried out.The convex hull algorithm was used to measure the contact angle of droplets in three-dimensional space,and the droplets were stratified along the z-axis perpendicular to the substrate.The charge density distribution and the number density distribution of anions and cations in the droplets with the change of height were calculated.The results show that the contact angle increased with the increase of ionic liquid droplet size under the condition of constant defect concentration of graphene oxide substrate.When the ionic liquid droplet radius remained unchanged,the contact angle decreased with the increase of graphene oxide substrate defect concentration.The intrinsic mechanism of the change of contact angle was the van der Waals potential and Coulomb potential interaction between ionic liquids and graphene oxide.These two kinds of potential drag droplets spreaded on the substrate surface,so that a layer of ion was adsorbed on the surface of graphene oxide,and the contact angle of the droplet decreased under the intermolecular interaction.The Coulomb force of anion and cation pairs of ionic liquids far away from the substrate was larger than the van der Waals force between the anion and cation pairs of ionic liquids and graphene oxide due to the dominant role of distance factor.The attraction and repulsion between anion and cation ions made the ionic liquid droplets tend to remain spherical,which hindered the wetting behavior of the droplets on the substrate surface and increased the contact angle.Due to the presence of oxygen-containing functional groups on the surface of graphene oxide,the interaction between graphene oxide and nano ionic liquid droplets was enhanced,and the surface was hydrophilic,which further indicates that the introduction of defects made the surface characteristics of graphene tend to be hydrophilic from hydrophobic.The research results in this thesis showed the wetting phenomenon of ionic liquid droplets on the surface of graphene oxide at the nanoscale.The internal mechanism of the change of wetting characteristics was explained by the contact angle and the distribution of anions and cations in the droplets.The wetting characteristics of nano-ionic liquid droplets on the surface of graphene oxide were preliminarily explored,which provided a theoretical basis for the application of ionic liquids and graphene oxide in the field of nano-electro-mechanical system. |
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