Study On Micro-morphology And Hydrophobic Characteristics Of Surface After Laser Processing

In nature,there are many animals and plants that have hydrophobic surfaces with thespecial micro-morphology and the wetting properties of these organisms has been a hot-spot in the field of bio-medicine.Because of its good corrosion resistance and bio-compatibility,titanium alloy has become a kind of widely used bio-medical material.By constructing the bionic micro-morphology on the titanium alloy substrate,the surface wettability could be changed from hydrophilicity to hydrophobicity.On the titanium alloy,Micro-morphology Molding Technology based on laser processing has the advantages of good flexibility,low cost and high repeat-ability,so the prospects for industrial applications are broad.In this paper,the conventional fiber-optic laser was used to process the surface of titanium alloy,the power density of the output laser varied with the change of processing parameters(output power,pulse frequency,pulse width,and scan speed).Thereby single grooved structures with different depth and width of cross-section were obtained on the surface of titanium alloy.Based on the experimental study of single grooved structures,this paper further constructed periodic grooved structures with different micro-structure dimensions.The cross section profile of titanium alloy surface after laser processing was measured and analyzed by needle drawing method,so the simplified discontinuous triangular wave could characterize the cross-sectional profile from the statistical point of view.Then combined with thermodynamics theory and minimum free energy theory of the system,the thermodynamic state model of the gas-liquid-solid three-phase system with parallel triangular wave cross-section profile was proposed,and the relationship between instantaneous contact angle and free energy of the system was established.The titanium alloy surfaces after laser processing needed to be modified by fluorosilane,then the static contact angle,advancing angle and receding angle before and after the surface modification treatment of each sample were measured from parallel and perpendicular to the texture direction.Experimental results showed that the micron-scale periodic grooved surfaces prepared by laser processing method had the significant an-isotropic wetting property and large contact angle hysteresis;After silanization,the surface of each sample had obvious isotropic super-hydrophobic wetting characteristics and small contact angle hysteresis.The main reasons for this phenomenon werethe energy barrier caused by different micro-structure size and the wetting state decided by chemical composition.In next step,the relationship curve between instantaneous contact angle and normalized free energy of the three-phase contact line at any position on the periodic grooved surface was generated by programming.Combining the experimental results and the model analysis results could verify the correctness of the theoretical model.In summary,by analyzing the change of contact angle and hysteresis of titanium alloy under different micro-structure size and chemical composition,this paper clarified that energy barrier caused by periodic grooved structures and wetting state affected by surface chemical composition were the main influencing factors of the wetting property of titanium alloy.And the wetting experiment on the surface of titanium alloy was explained by analyzing the thermodynamic state model of the gas-liquid-solid three-phase system on periodic grooved structures.The results of this study could provide theoretical and technical supports for the design and evaluation of solid surface micro-structures under industrial applications.It could also provide a reference for the popular super-hydrophobic surface processing scheme.