A Molecular Dynamics Simulation Study On The Wettability Of Hairy Surface

The regulation of the wettability of the solid surface has been widely concerned.By adjusting the wettability of the material surface,we can obtain self-cleaning,antibiological pollution materials.By adjusting the chemical properties of the solid surface and the topological structure of the material surface,the wettability of the surface can be greatly changed.The biomimic study of hydrophobicity of lotus leaves shows that increasing the roughness of the surface can greatly improve the contact angle.In the observation of nature,it has been found that for certain species of plants,droplets can slide from the surface covered with the hairs,thereby giving the leaf a self-cleaning ability.The unique leg structure of the water strider yields that it can walk on the water.In both of the above examples,plant leaves and insect legs are covered with tiny hairs.Based on these biomimic studies,surface materials covered with hairy structures are prepared in the laboratory.However,the current mechanism of wettability of hairy surface material is not clear and controversial.In this way,the factors that clearly affect the wettability of the hairy material provide an effective way to design a material that is resistant to droplet infiltration.In addition,by improving and optimizing the geometry of the surface roughness,the hydrophobicity of some originally non-hydrophobic materials can be improved in quality and can even resist the infiltration of low surface tension liquids(such as certain oils).In this paper,the factors affecting the wettability of surface grafted with hairs of different shapes,such as length,graft density and hardness,are studied by means of coarse-grained molecular dynamics method.A computer algorithm for analyzing the hairy structure is proposed.The main contents of this thesis include:(1)The factors influencing the surface wettability of linear hair are studied.By adjusting the graft density,length and rigidity of hair,the non-wetting properties of the hairy surface can be greatly improved.When the droplets stay on the top of the hair,the hair will spontaneously cluster to form hair bundles.We plot the phase diagram of the droplet contact angle with the length of the hair and the graft density.In addition,we also reveal the relationship between the orientation of hairs and the contact angle.Unlike a hard solid surface and a solid surface with purely flexible chain,the hair itself has adjustable rigidity.From this point of view,our simulation of hairy surface materials fills the gap between the simulations of flexible solid surface and rigid solid surface.(2)The influence of the geometrical structure of different hairs on the wettability is studied,and a set of process for characterizing surface roughness is put forward.We model comb-like,y-shape,and linear hairs,respectively.In the simulation,we observe that hairs will self-assemble to form different surface structures,such as network structures,hair bundles and so on.According to the hair rigidity,hairy surfaces can be divided into two categories,namely,rigid hair and flexible hair.For rigid hair,the contact angle mainly depends on the top layer density of hairs;for the flexible hair,the contact angle mainly depends on the surface property below the droplet,such as the strength of the hair bundle/the network structure,the topology of hair bundles and the nonwetted area.In order to characterize the network structure,we develop a set of algorithm to calculate the number of junctions in the top layer of network structure.In this work,we find that in some specific cases,hair can spontaneously form a partial re-entrant structure.Finally,we also emphasize that the surface that can strongly resist droplet infiltration does not necessarily have a very high contact angle.(3)The effects of the rigidity,arrangement and polydispersity of loop on the wettability are studied.The loop structure is analyzed by our previous method.Regularly arranged hairs can form a regular groove-like structure on the surface,while the randomly arranged hairs form a more rugged structure.For monodispersed loop hair,we analyze the effect of hair rigidity on the surface wettability.We divide the grafted surfaces into the rigid loop and the flexible loop.It is found that a partial re-entrant structure is formed on the surface of the material,which helps to enhance the non-wettability of the material.We find that for relatively flexible hair,the loop structure is a good way to enhance the nonwettability.For the surface grafted with the polydisperse hair,the wettability is attributed to the ability of the long hair to support the droplets.