Superhydrophobic Metallic Surfaces Fabricated By Laser Ablation And Their Properties

There are many surfaces of animals and plants exhibiting superhydrophobicity in nature,for example,water droplet can easily roll off from the lotus leaf under the action of gravity or wind;water strider can easily walk on the water surface;and butterfly can freely fly in rainy weather.Inspired by nature,many artificial superhydrophobic surfaces have been developed.Generally,low surface energy and rough surface structure are two indispensable factors to obtain superhydrophobic surfaces.Therefore,fabricating superhydrophobic surfaces on intrinsically hydrophilic metallic substrates usually demands two steps: firstly,creating rough structure on metallic substrates,which can be achieved by anodization,electrodeposition,laser ablation,and so on;secondly,chemically modifying the structured surfaces with low-surface-energy reagents such as fluorosilane,chlorosilane and thiol.However,the surface chemical modification methods suffer from some limitations such as high fabrication cost,complex operation,and environmental risk.Laser technology has been attracted intense interest in many fields such as optic,medic,and communication since it came out.In the past decades,laser-ablation technique has gained increasing attention in the fabrication of the superhydrophobic metallic surfaces because of its various advantages,including excellent ability to construct versatile micro/nano-scale hierarchical structures,applicability in a large range of materials,high safety and easy manipulation for operators,low cost and fast speed for sample preparation,etc.Moreover,some researchers have found that the wettability of the laser-ablated metal surfaces can change from superhydrophilicity to superhydrophobicity after being stored in air for several weeks or being heated at lower temperatures for several hours.However,the mechanism of such wettability transition is still controversial,and the properties of the superhydrophobic surfaces obtained by such wettability transition have been seldom studied.To solve above problems,in this paper,the effect of the heat treatment on the wettability of the laser-ablated metal surfaces were systematically studied,the reasons of the variations in wettability during heat treatments were analyzed,and the related properties,including the wetting transition,underwater superwetting properties,corrosion resistance and adhesion of the superhydrophobic metal surfaces were explored.The main works and corresponding results are as follows:1.micro/nano-scale hierarchical structure was constructed on brass substrate by laser ablation.The effect of different heat treatments on the wetting properties was experimentally examined.The results showed that the laser-ablated brass surface change from superhydrophilicity to superhydrophobicity during the low-temperature heat treatment,while the surface keep superhydrophilicity during the high-temperature heat treatment.When lowtemperature and high-temperature heat treatments were alternately performed on the laser-ablated brass surface,a reversible wettability transition between superhydrophilicity and superhydrophobicity was observed on the surface.Analysis of surface compositions revealed that the adsorption of airborne organic compounds and the decomposition of the adsorbed products alternately take place on the laser-ablated brass surface during alternate heat treatment,which leads to the reversible change in the substrate wettability(intrinsic contact angle).Analysis of surface structure demonstrated that the micro/nano-scale hierarchical structure greatly reduces the width of the Wenzel region and thus strongly magnifies the substrate wettability.The reversible change in the substrate wettability and the magnifying role of the micro/nano-scale hierarchical structure in the substrate wettability are responsible for the observed reversible wettability transition on the laser-ablated brass surface.Furthermore,the alternate heat treatment was also performed on laser-ablated aluminum,304 stainless steel and titanium surfaces,and the reversible wettability transition between superhydrophilicity and superhydrophobicity was also observed on these surfaces.2.The effects of low-temperature heat treatment on underwater bubble wettability and underwater oil wettability of laser-ablated brass surface were studied.It was found that four underwater superwetting properties,including underwater superaerophobicity,underwater superaerophilicity,underwater superoleophobicity and underwater superoleophilicity can be obtained on brass surface via laser ablation and low-temperature heat treatment.The superwetting properties are related to the wetting states of the laser-ablated brass surface.When the superhydrophilic laser-ablated brass surface was immersed in water,the gaps in the micro/nanoscale hierarchical structure were occupied by water which is incompatible with oil and air bubble,as a result,the surface exhibit underwater superaerophobicity and underwater superoleophobicity.When the superhydrophobic laser-ablated brass surface was immersed in water,the gaps in the micro/nano-scale hierarchical structure were occupied by air,which endows the surface with underwater superaerophilicity and underwater superaerophilicity.Moreover,the underwater bubble superwetting properties and underwater oil superwetting properties can be applied in the transport of underwater bubbles and oil-water separation,respectively.3.Electrochemistry tests were performed on superhydrophobic and superhydrophilic laserablated brass surfaces and polished brass surface to study the corrosion resistances of these surfaces,and the durability of the corrosion resistance of the superhydrophobic laser-ablated brass surface was studied by immersion test.The results of the electrochemistry tests showed that fabricating superhydrophobic surface via laser ablation followed by low-temperature heat treatment can markedly improve corrosion resistance of the brass.The good corrosion resistance of the superhydrophobic laser-ablated brass surface arises from its Cassie-Baxter wetting state.In this wetting state,the gaps in micro/nano-scale hierarchical structure were occupied by an air film which provides an extra barrier and decreases the solid-electrolyte contact area.The result of the immersion test showed that the corrosion resistance of the superhydrophobic laser-ablated brass surface is always better than that of the polished brass surface during the long-term immersion,which demonstrates that the corrosion resistance of the superhydrophobic laser-ablated brass has a good durability.4.Superhydrophobic surfaces with different water adhesions were obtained by creating micro-scale grid structures with different sizes on brass substrate via tuning the laser scanning interval followed by low-temperature heat treatment,and the corrosion resistances of these surfaces were studied by performing electrochemistry tests.Analysis of surface structure revealed that the different water adhesions of these superhydrophobic laser-ablated brass surfaces result from the different solid-liquid interfacial area fractions.The surface with smaller scanning interval has the larger proportion of the gaps in the rough structure,which efficiently decreases the solidliquid interfacial area fraction,leading to the lower water adhesion.In contrast,the surface with larger scanning interval has larger solid-liquid interfacial area fraction and has higher water adhesion.The results of electrochemistry tests showed that the surface with lower water adhesion has better corrosion resistance,which stems from its smaller solid-electrolyte contact area.