Preparation And Characteristics Of Biomimetic Superhydrophobic Metallic Surfaces

Wettability with a liquid is one of important properties of solid surfaces, which is go-verned mainly by surface rough structures and chemical compositions. Recently, with thedevelopment of wetting theory and the improvement of processing technique, many specialwettability including superhydrophobicity, superhydrophilicity and superamphiphobic haveattracted much interest and become a research hotspot now in the fields of individual sub-jects and interdiscipline. Lotus leaf has always been an archetype for superhydrophobicand self-cleaning surfaces. Water droplets on lotus leaf can move and roll off easily withdust particles and surface contaminants. This phenomenon, known as “lotus effect”, is at-tributed to unique micro/nano-scale hierarchical structures and hydrophobic epicuticularwax. Inspired by lotus leaf, various techniques have been developed to fabricate superhy-drophobic and self-cleaning surfaces from different substrate materials. Nevertheless, itstill remains a great challenge to fabricate such surfaces from inherently hydrophilic mate-rials due to dual limits of fundamental theory and manufacture technique. It is well-knownthat metal with high surface energy is typical inherently hydrophilic material. Almost allliquids tend to spread out on and thereby wet metallic surfaces. Generally, preparation ofrough surface and subsequent coating of the surface with low surface energy materials areessential processes in fabricating metallic superhydrophobic surfaces. In spite of consider-able success, the chemically modified layers on metal substrates are always fragile and of-ten deteriorate in harsh environments, accompanied by the loss of superhydrophobicity. Inaddition, the chemically modified layers would affect some intrinsic properties of metallicsurfaces such as electrical conductivity. Therefore, it is of both fundamental and practicalsignificances to fabricate superhydrophobic and self-cleaning metallic surfaces withoutusing any surface chemical modification.In this paper, a series of Cu films with different surface structures were fabricated onsmooth Cu foils substrates by using an improved one-step electric brush-plating technique with careful control of the brush-plating processing parameters. The surface geometricalmorphologies, crystalline structures and chemical compositions of the Cu films were cha-racterized systematically by the scanning electron microscope (SEM), field emission scan-ning electron microscope (FESEM), X-ray diffraction (XRD), energy dispersive X-rayspectrometer (EDS), X-ray photoelectron spectroscopy (XPS), etc. The effects of the vary-ing surface structures of the Cu films on the surface wetting state, surface wettability, sur-face adhesion and impact dynamics of water droplets were investigated by performingstatic and dynamic wetting tests, in conjunction with theoretical calculations. The main re-sults are listed as follows:1) Five Cu films with different surface structures were fabricated mainly by changingbrush-plating voltage (2.5~4.5V) and other processing parameters including current den-sity (0.6~2.2mA/mm2), shifting rate of stylus (1~8m/min), contact pressure between ca-thode and anode (1~12KPa) and brush-plating time (15~40min), which are respectivelyreferred to as Cu films I, II, III, IV and V in the order of low-to-high brush-plating voltage.The SEM and FESEM observations and XRD analysis showed that the Cu films exhibittypical lotus-like three-level hierarchical structures composed of micro-protrusions, sub-micro-bumps and nano-grains. The obvious evolution of the micro-scale structure but notthe submicro-and nano-scale structures depends strongly on the brush-plating voltage. TheEDS and XPS analysis revealed that the Cu films were only oxidized considerably slightlyin air. The surface static water contact angle and dynamic contact angle hysteresis mea-surements showed that in the absence of any surface chemical modification, Cu films I andII show superhydrophobicity and low adhesion, and Cu films III, IV and V show stronghydrophobicity and high adhesion. The surface hydrophobicity of the Cu films successive-ly (I→V) decreases, while the surface adhesion goes in the opposite direction.2) The water submerging test, water droplets rolling test and self-cleaning testshowed that Cu film I exhibits robust superhydrophobicity and good self-cleaning ability,which may arise from the air pockets or film trapped between the Cu film and water. Anal-ysis based on an alternative wetting mechanism map revealed further that the robust su-perhydrophobicity of Cu film I should be attributed to the surface hydrophobization role caused by the nano-scale structure and the solid-liquid contact area reduction role causedby the micro-and submicro-scale structures, which together give rise to the stable Cassiesuperhydrophobic state. Further analysis based on the variation of calculated system ener-gy revealed that the submicro-and nano-scale structures can prevent effectively waterdroplets penetration by pinning the liquid-air interface inside the micro-scale structure,thus leading to a weakened surface adhesion which together with the robust superhydro-phobicity endows Cu film I with good self-cleaning ability. Such wetting mechanism isalso suitable for the natural lotus leaf with similar three-level hierarchical surface struc-tures.3) Based on the classical Wenzel and Cassie wetting theories, the effects of the vari-ation of micro-scale structure on the surface wetting state, surface wettability and surfaceadhesion of the Cu films and the theoretical condition for the existence of the stable Cassiesuperhydrophobic state on rough solid surfaces were investigated by theoretical calculationcompared with experimental observations. It was demonstrated that increasing aspect ratioand spacing factor of the micro-scale structure will lead to the Wenzel-to-Cassie, hydro-phobicity-to-superhydrophobicity and high-to-low adhesion transition. To obtain the stableCassie superhydrophobic state, the basal surface equilibrium water contact angle and as-pect ratio should be as large as possible and the spacing factor should be limited within aspecific range.4) According to the performance of water droplets impacting on the Cu films at awide range of impact velocity (0.14~3.13m/s) or Weber number (0.27~133.20), it wasfound that the dynamic water-repellency of the Cu films becomes successively (I→V)weaker, and especially compared with natural lotus leaf and some other chemically mod-ified superhydrophobic surfaces, bare Cu film I shows considerably robust dynamic su-perhydrophobicity. Analysis with the help of a dynamic impact schematic constructedbased on the pressure balance relation revealed that such wetting behavior arises from thecooperation of the unique three-level hierarchical structures, i.e. the damping role of mi-cro-scale structure, the delaying role of submicro-scale structure and the surface hydro-phobization role of nano-scale structure. Meanwhile, the variations of the damping and de- laying roles and the surface wetting state and wettability caused by the varying micro-scalestructures should be the main reason for the successively (I→V) reduced dynamic wa-ter-repellency of the Cu films.