Study On The Wettability Influencing Factors Of Micro-structure Hydrophobic Surfaces

Wettability is an important property of solid surface,many physical and chemical processes are all closely related to the wettability of the surface.It plays a critical role in human production and life.Hydrophobic surface can be widely used in architecture,biomedicine and other fields because of its excellent properties such as self-cleaning and anti-wear.In recent years,inspired by the "Lotus Effect",researchers have been working to achieve better hydrophobic surfaces.In this paper,theoretical analysis is laid the foundation of the whole discuss,and the influence factors of wettability on hydrophobic surface with microstructure are studied by simulation and experimental methods.The main work and conclusions are as follows.(1)Taking a droplet on the solid surface of micro-pillar as the object to study,based on the geometric conditions and force balance principle of the physical model under composite and non-composite wetting conditions,the mathematical relationship of the geometric parameters of the micro-pillar structure(side length,spacing,the height)and surface apparent contact angles is calculated.Based on the principle of thermodynamics,the relationship between the geometric parameters of the micro-pillar and the surface apparent contact angle,even the relative free energy of the wetting system is established.The effects of the intrinsic contact angle and geometric parameters on the apparent contact angle in the composite and non-composite wetting states are analyzed,and the two hydrophobic state transitions of the hydrophobic surface were discussed based on the geometric parameters of the micro-pillar.The results show that the apparent contact angle in the composite wetting state depends on the intrinsic contact angle of the material,the spacing and the side length of the micro-pillars,and the apparent contact angle in the non-composite state is also related to the height of the micro-pillar.In the composite wetting state,increasing the intrinsic contact angle and the spacing between the micro-pillars or decreasing the side length can increase the apparent contact angle of the surface and enhance the hydrophobic properties of the surface.In the non-composite wetting state,increasing the height of the micro-pillar also increases the apparent contact angle.From the thermodynamic equations,there are critical heights and critical side lengths,which make the relative surface free energy of non-composite and composite states equal.When the height of the micro-pillar is less than the critical height or the length is longer than the critical side length,the relative free energy of the non-composite state is smaller than that of the composite state,and the system is relatively stable in the non-composite state.When the height is greater than the critical height or the length is less than the critical side length,the relative free energy of the non-composite state is larger than that of the composite state,and the system is in a more stable state in the composite state.(2)Using the phase field method in the numerical simulation software COMSOL Multiphysics,the two-phase flow of droplets on the micro-pillar rough surface is simulated.The effects of the intrinsic contact angle of the surface,the aspect ratio of the micro-pillar,the depth-to-diameter ratio,and the initial velocity of the droplet on the surface wettability and the dynamic process are discussed.Among them,the numerical values of the apparent contact angles obtained from the simulation are basically consistent with the results of the relevant theoretical analysis in Chapter 2.The dynamic wettability simulation part points out the transition critical conditions of the composite wet state and the non-composite wet state:the critical aspect ratio and the critical depth-to-diameter ratio enable the droplets to maintain the composite wetting state on the micro-pillar surface.When the height of the micro-pillar is 35μm,the critical aspect ratio is 1;when the spacing of the micro-pillar is 50μm,the critical depth-to-diameter ratio is 0.7.During the impact of the droplets on the surface,a larger aspect ratio means a larger spread and retract rate,and also a larger maximum spread diameter,but a larger depth-diameter ratio corresponds to a smaller spread and retraction rate and smaller maximum spread diameter.Different initial velocities will affect the dynamic spreading process of the droplets on the surface.For a lower height of the micro-pillar surface,larger velocities will cause the droplets to infiltrate,and the droplets will be transformed from the composite wetting to the non-composite wetting state on the surface.For a high-height micro-pillar surface,the infiltration rate is greater than the rebound rate,and droplets are prone to rebound on solid surfaces.Combined the conclusions of Chapters 2 and 3,it is concluded that when the intrinsic contact angle of the solid material is 120° and the droplet volume is 1 μL,the ideal micro-pillar spacing,side length,and height combination are 50 μm,50 μm,and 50 μm.(3)The surfaces of polystyrene(PS)fibers with different diameters and morphology and PS microspheres with different densities are prepared by electrospray technique,and the effects of fiber diameter,fiber morphology,and microsphere density on the wettability of PS film are analyzed.The experimental results show that when the electrospun products are smooth fibers,as the average diameter of the fibers increases,the static contact angle decreases.In the range of produce where the electrospinning product is beaded fiber,as the mass fraction of the polymer solution increases,the number of beads decreases,the beaded shape tends to be flat,the roughness decreases,and the static contact angle decreases.When the mass fraction of polystyrene solution is less than 5%,the electrospun products are microspheres.As the density of microspheres on the fibers increases,the surface roughness increases,and the contact angle of the surface gradually increases.The local microstructure of the prepared PS film can be compared with the micro-pillar structure in Chapter 2.With the decrease of the characteristic size of the PS film,i.e.,the size of the micro-pillar structure,the entrapped air between the microstructures increases and the hydrophobicity increases.