Research On Processing And Wetting Characteristics Of Surface Special-shaped Microstructure

Surfaces with hygrophobic capabilities have broad and important applications in engineering.For example,in the field of condensation heat transfer,hygrophobic surfaces allow the liquid to disengage rapidly to reduce thermal resistance.In various industries,there are a large number of low surface energy working fluid for which traditional surface hydrophobic treatment is not applicable due to their high wettability.Special-shaped microstructures are widely used to solve these problems because of their ability to improve the force characteristics of liquid on the surface and to retain a large number of air cavities,so the study of surface special-shaped microstructure processing and wetting characteristics is of great importance.In this thesis,from the perspective of engineering applications,three different surface processing technologies for special-shaped microstructures are studied.A metal powder sintering process was used.In order to solve the problem that the special-shaped structure cannot be extracted from the mold in the traditional sintering process,the special-shaped microstructure surface is divided into two parts and sintered separately,and then the two are precisely positioned by nested molds and combined by secondary sintering to finally obtain the metal powder secondary sintered shaped microstructure surface.By investigating the effect of copper powder particle size on its shrinkage rate and demolding performance,a divisional filling scheme is determined to ensure the dimensional accuracy of the microstructure surface.Two pore filling schemes were compared,and paraffin filling was determined to avoid liquid drawdown from the pores of the sintered body.The hydrophobic properties of the surface were characterized using droplets with different surface tensions,and the effect of different geometric parameters on its wettability was investigated.The experimental results show that the surface can keep the surface tension of 30.69 m N/m of 40% ethanol solution as a minimum in the hydrophobic state.Its contact angle decreases with the increase of the top diameter and the spacing of the structure,and the larger structure produces significant changes in the contact angle values of the large size droplets,but the overall trend remains the same.The ion-etching metallic copper-based special-shaped microstructure surface was processed,and its structure size was further reduced.In order to solve the problem that the special-shaped structure is difficult to be formed in one step,a multi-step processing process of machining the bottom of the structure,diffusion welding the copper sheet to form the top substrate of the structure,and ion beam etching to release the top structure was used.Among them,the machined substrate and mask template were co-designed,and the alignment accuracy of the structure was ensured by using positioning holes.The effect of welding pressure on the deformation of the structure was investigated,and it was confirmed that a welding pressure of 1 MPa could ensure the welding quality of the sample under the condition of low deformation.Finally,fluorosilane modification was used to further reduce the surface energy of the material.Its hydrophobic ability was characterized and the effect of different arrangements on its wettability production was studied.The experimental results show that the surface can keep the hydrophobic state of60% ethanol solution with a surface tension of 26.72 m N/m at the lowest.The surface of the lattice structure has a higher contact angle,and the channel structure has anisotropic wettability and can support the liquid with lower surface energy to keep the hydrophobic state.With lithography technology,a smaller size of lithographic silicon-based specialshaped microstructure surface is successfully fabricated with a channel arrangement structure.And thanks to the difference of etching rate between silica and silicon,the special-shaped structure has a doubly reentrant feature.The wettability experiments show that the surface can raise the contact angle of 60% ethanol solution with surface tension26.72 m N/m from 36.36° to 152.34° in the direction of parallel channel.The contact angle increases gradually with the increase of structure spacing and decreases with the decrease of droplet surface energy.In this process,the surface with larger structural spacing takes the lead in wetting,reflecting the contradictory characteristics of the contact angle value and the ability to maintain the hydrophobic state.Compared to the first two surfaces,the contact angle is less affected by the droplet volume.The sliding angle also has anisotropic characteristics,decreasing with the increase of structural spacing and droplet volume.