Preparation And Anti/De-icing Mechanism Of The Superhydrophobic Surface On Ti6Al4V

During the process of the aircraft flying,ice formation and accumulation on the surfaces of aircrafts will take place due to the supercooled water droplets gathering in the clouds,endangering the safety of the flights.Currently,many popular strategies of anti-icing and de-icing,including mechanical,clectrothermal,and liquid hybrid methods,are based on the thoughts of breaking and melting ice.However,some necessary anti/de-icing devices increase the weight of aircraft and reduce the fuel efficiency,bringing many disadvantageous factors to the design and manufacture.Also,the safety and reliability of aircrafts will greatly reduce due to the surface metal being under the conditions of thermal cycle and mechanical vibration.Herein,based on the technology of surface modification,this thesis utilized the intrinsic properties of materials to design and fabricate the anti-icing surfaces,simultaneously exploring the application potential in the aircraft industry.According to the ice formation conditions on the surfaces of aircrafts,the candidate anti-icing surfaces should meet the three demands:?1?the high water repellency;?2?the high icing-delay performance;?3?the low ice adhesion strength.Thus,we firstly fabricated the special nonwetting surfaces containing the specific micro-nanoscale hierarchical structures.Subsequently,based on the special interface nonwetting state caused by the micro-nanoscale structures,we mainly focused on the investigations of analyzing icing-delay performance,ice adhesion strength,and the impact behaviors of dynamic droplets at subzero conditions.Also,the present work emphasized the investigations of inhibiting ice nucleation and growth,and the reducing mechanism of the ice basing on the interface between ice and surface.The thesis obtained the five main conclusions as follows:1)Based on the microscale concave-convex structured surfaces manufactured by sand blasting,we designed and constructed three kinds of nanostructures?nanotube,nanowire,and nanomesh?by means of anodic oxidation,hydrothermal treatment,and acid/alkaline etching technique,respectively.Subsequently,after modifying with FAS-17,we obtained the superhydrophobic sample surfaces.Under the action of secondary nanostructures,the superhydrophobic surfaces trapped a large amount of air pockets underneath the droplets,forming the stable Cassie-Baxter wetting model.It also resulted in a very small fractional area of the actual solid-liquid contact area at the apparent contact interface and a higher apparent contact angle.Additionally,compared with nanotube or nanomesh structure,nanowire structure seemed to be more effective to improve the superhydrophobicity?the apparent contact angle reaching 161° and the sliding angle of approximately 2°?,and possessed the ability to compel the impact droplet to bounce off the solid surface within 12 ms.2)This thesis proposed a new strategy of characterizing the contact behaviors and contact time of the impact droplets on the superhydrophobic surfaces to further evaluate the water repellency.Also,we concluded that the contact behavior and contact time mainly depended on the dynamic contact angle or contact angle hysteresis instead of the static apparent contact angle,according to the research results on the relationship between the contact time and the wetting hysteresis of the solid surfaces.Furthermore,we designed some complex ridge macrotextures on the superhydrophobic surface,and through more properly altering the water droplet hydrodynamics,compelled the retracting process of the impact droplet to be taken place ahead of schedule and shortened the contact time.Also,it could be found that the contact time was shortened to a theoretically limited value of 5.4 5.5 ms?i.e.,the time required to spread out to maximal deformation?under the action of the “cross-shaped” macrotexture.3)Based on the optimized micro-nanoscale composite structured?microscale concave-convex and TiO₂ nanowire?superhydrophobic surface,this thesis focused on the systematic characterizations and analyses on icephobic potential of the superhydrophobic surfaces at lower temperatures of-10 °C,-20 °C,and-30 °C,including static analysis of ice adhesion strength and icing-delay time,and the dynamic contact process of an impact droplet.It could be concluded that the icing-delay time of the droplet on the superhydrophobic surface reached approximately 750 s at-10 °C,increasing dozens of times compared with the smooth matrix materials.Also,the ice adhesion strength was reduced to only 80 KPa?Internationally recognize that the value below 100 KPa for an ideal anti-icing surface?.In the dynamic droplet impact and rebound assay on the superhydrophobic surface,although the contact time was extended with the reduced temperature,the water droplet could always bounce off before freezing regardless of temperatures?2³ orders of magnitude?,owing to the robust water-repellency.These investigations demonstrated that the micro-nanosale composite structured superhydrophobic materials were a promising anti-icing surface,which had significant potential for application.4)We analyzed the action mechanism of the secondary nanostructure in the wetting model transition process from Wenzel wetting model?microscale concave-convex structured hydrophobic surface?to Cassie-Baxter wetting model?micro-nanoscale hierarchical structured superhydrophobic surface?.It could be found that a thin layer of air was formed around the hydrophobic nanostructure,when the spacing distance of the nanostructure was smaller than a certain value d'?'d?28?100nm for water?,the thin air layers would attract each other to form a larger continuous air layer.Combined with a certain size of microscale structure,a large number of air pockets could be trapped to form the stable Cassie-Baxter wetting model.Finally,the function mechanism of the superhydrophobic surface inhibiting the ice nucleation and growth was investigated,indicating that the trapped air pockets in the micro-nanoscale hierarchical structures resulted in a small actual contact area of approximately 0.025 mm2?0.35% of the smooth matrix materials?,lastly leading to a lower nucleation rate?at-25 °C,reducing approximately 19 orders of magnitude compared with the untreated substrate?.Meanwhile,the air pockets served as the heat insulation layer to reduce the ability of the water molecules moving to the interface of crystal nucleus and water,also causing a lower growth velocity of the ice layer.5)This thesis was also to verify the de-icing capacity of the superhydrophobic surface under the conditions of wind field and thermal field,expecting to promote the practical applications.It could be found that under the condition of wind field,the micro-nanoscale hierarchical structured superhydrophobic surface displayed a remarkable deicing property?no matter what the freezing temperature was?,showing the time required for blowing away the ice on the surface being least.This mainly attributed to the composite contact interface of ice-air and ice-solid,and the self-assembled hydrophobic group membrane preventing the production of hydrogen bonding between the ice and solid surface.However,the composite contact interface also brought the disadvantages and caused a poor electrothermally melting ice property owing to the trapped air pockets reducing the heat transfer efficiency.Therefore,cooperating with a wind field deicing,the anti-icing superhydrophobic surface can exhibit a promising the practical application potential,and also shows a robust durability under the condition of the continuously icing/wind-deicing cycles.