Fabrication And Application Of Extreme Wettability Surfaces On Engineering Metal Materials

The extreme wettability surface is the surface with liquid contact angles less than10°or larger than150°. This surface has a great application value in the area of military, communication, and oil exploitation. The obstacle of its application is lacking safe, environment-friendly and highly effective method to fabricate surface microscopic structures. Based on the deep analysis of the related theory of wettability and research status of extreme wettabitlity surfaces on engineering metal materials, according to the mechanism and characteristics of non-traditional machining technology, such as chemical deposition, chemical etching, electrochemical anodic etching, and electrochemical anodization, etc, a series of basic theoretical and experimental study on construction and control of micro/nanometer-scale microscopic structures were carried out in the dissertation by using self-designed device. The evolvement rule of surface microscopic structures under different non-traditional machining technology was observed and summarized. Combining with the modification of low surface energy materials, the extreme wettability surface was fabricated. In addition, based on the obtained extreme wettability surface, the mechanism and rule of the extreme wettability surface on the application of anti-icing and frosting and oil/water separation was studied, providing the theoretical and technological foundation for the application of the extreme wettability surface on the anti-icing and frosting of key parts of plane and recover of large area oil spill on sea. The main content is as follows,(1) Fabrication of extreme wettability surface on Al and stainless steel materials by chemical deposition and chemical etching using the aqueous solution containing Cu2+and Cl-. Based on the theory that the metal with lower electrode potential can react with the salt solution of metal which has higher electrode potential and the latter metal can be reduced to the elemental form and precipitated out and deposited on the former metal, the influence of immersion time in the aqueous solution containing Cu2+and Cl-on surface microscopic structures, chemical composition, and wettability was studied. The results show that, after chemical deposition, the microscopic structures composed of micrometer-scale particles, submicrometer-scale dendrites and nanometer-scale crystals were constructed on Al materials, showing superhydrophilic extreme wettability; the micro/nanometer-scale leaf-like structures were constructed on stainless steel materials, showing superhydrophilic extreme wettability. After the following modification by low surface energy materials, all the aforementioned surfaces showed superhydrophobic extreme wettability with water contact angles larger than164°and sliding angles less than10°. In addition, the deposited Cu on Al materials can be removed by ultrasonic cleaning. It was found that the whole Al surfaces were chemically etched to form irregular micro/nano-meter scale rectangular concave and convex Al structures and showed superhydrophilicity. After the following perfluorooctyltriethoxysilane (FAS) modification, the etched Al surfaces showed superhydrophobic extreme wettability. This method has the advantages of high processing efficiency and low requirement for device, suitable to the fabrication of extreme wettability surfaces on small-area metal materials with complex shape.(2) Fabrication of extreme wettability surface on Al, Mg alloy, and Cu materials by electrochemical anodic etching using neutral electrolyte. Based on the theory of uneven electrochemical dissolution of anodic metal, the influence of substrate materials and electrolyte composition on surface microscopic structures and wettability was studied. The formation reason of the special surface microscopic geometric morphology was determined. Based on this, the method and experimental device used to fabricate large-area extreme wettabitlity surfaces on engineering metal materials were designed. The influence on inter-electrode gap on surface macroscopic morphology and wettability was studied. The results show that, depending on the preferential anodic dissolution of grain boundaries and dislocations and the abscission of some grains for Al under an applied electric field, the micrometer-scale rectangular concave and convex structures were constructed on Al materials by electrochemical anodic etching using NaCl electrolyte or NaClO3electrolyte; depending on the preferential anodic dissolution of a phase, the negative difference effect corresponding to the hydrogen production on anode, and the abscission of some un-corroded protrusion structures for Mg alloy under an applied electric field, the micrometer-scale pits and protrusions and nanometer-scale mastoids were constructed on Mg alloy materials by electrochemical anodic etching using NaCl electrolyte; depending on the anodic passivation of Cu in the solution containing Cl-under an applied electric field, the submicrometer-scale potato-like and nanometer-scale grain-like structures were constructed on Cu materials by electrochemical anodic etching using NaCl electrolyte. After electrochemical anodic etching, Al and Mg alloy materials showed superhydrophilic extreme wettability and Cu materials showed hydrophilicity. After the following FAS modification, all the aforementioned surfaces showed superhydrophobic extreme wettability with water contact angles larger than164°and sliding angles less than10°. This method has the advantages of high processing efficiency and small harm to environment and operators, suitable to the fabrication of extreme wettability surfaces on large-area metal materials with planar shape.(3) Fabrication of extreme wettability surface on Al materials by electrochemical anodization using Na3PO4electrolyte. Based on the theory that the anodization of Al easily occurs in an alkaline solution under an applied electric field, the influence of anodization time on surface microscopic structures, wettability, and micro-hardness was studied. The results show that, in the electrochemical anodization process, the porous honeycomb-like structures were formed first on the Al surfaces and then transformed into nanowire-like structures, showing superhydrophilic extreme wettability. After the following FAS modification, the surfaces showed superhydrophobic extreme wettability with water contact angles of168°and sliding angles of1°. The roughness of the resulted surfaces was only Ra0.409μm, far lower than the surfaces obtianed by other methods. Besides, the microhardness of the resulted surfaces was11.6times of common Al surfaces. Low roughness and high hardness can improve mechanical property obviously.(4) Fabrication of superoleophobic extreme wettability surface on Al materials by electrochemical anodic etching and boiling water or [Ag(NH3)2]+solution immersion. Based on the micrometer-scale microscopic structures obtained by electrochemical anodic etching using NaCl electrolyte, the nanometer-scale structures were constructed by boiling water or [Ag(NH3)2]+solution immersion. The influence of immersion time and surface micro/nanometer-scale structures on wettability of liquids with different surface tension was studied. The mechanism of the obtained superoleophobicity was explained by reentrant structures theory. The results show that, after electrochemical anodic etching and boiling water or [Ag(NH3)2]+solution immersion, the micrometer-scale rectangular concave and convex structures and nanometer-scale needle-like structures or nanometer-scale grain-like structures were constructed on Al materials. Nanometer-scale structures can effectively transform the micrometer-scale non-reentrant structures into reentrant structures which are needed by superoleophobicity. After the following perfluorooctanoic acid modification, superoleophobic extreme wettability was obtained. The contact angle of hexadecane with surface tension of only27.5mN/m on the surface obtained by boiling water immersion was157.7°. Good superoelophobicity can effectively prevent surfaces from oil pollution.(5) Based on the obtained superhydrophobic surface on Al materials and the self-designed device of refrigeration, the appearance time, growth rate, and freezing time of condensation droplets on superhydrophobic surfaces and common Al surfaces was observed and studied using microscope. The mechanism of anti-icing and frosting of superhydrophobic surface was analyzed. The results show that the big contact angle of superhydrophobic surface increased the thermodynamic barrier of liquid phase, reduced the activation rate of liquid core, delayed the cooling time of liquid droplets. Compared with the common Al surfaces, the late appearance time, low growth rate, and late freezing time of condensation droplets on superhydrophobic Al surfaces indicate that superhydrophobic surface has good property of anti-icing and frosting. (6) Based on the obtained extreme wettability surface on stainless steel mesh and the self-designed device, the conditions and mechanism of the successful separation of light-oil/water mixtures and heavy-oil/water mixtures by traditional pouring-type, gravity-driven method was studied. The results show that only the superhydrophilic mesh that was pre-wetted with water could successfully and completely separate light oil and water (e.g. mixture of hexadecane and water); while for a heavy-oil/water mixture (e.g. mixture of chloroform and water), either the superhydrophilic mesh that was pre-wetted with oil or the dry superhydrophobic mesh provided sustained and complete separation.(7) In order to solve the disadvantages of present oil/water separation methods using extreme wettability surfaces, a surface tension-gravity double driven oil/water separation method and device was invented based on the obtained superhydrophobic stainless steel mesh. Oil collection efficiency, oil collection velocity and the influence of pollutants on separation were studied. The results show that the invented surface tension-gravity double driven oil/water separation device collected different types of oil with efficiency higher than94%under the action of surface tension and gravity. This method doesn’t need mechanical handling (squeezing/compression) and pre-collection of oil/water mixtures, and has a long working life, suitable to the in-situ, sustained collection of large-area floating oil on sea.