Superhydrophobic surfaces are corrosion-resistant and have excellent anti-icing, anti-fogging, and anti-fouling properties. They have the potential for broad applications in machinery manufacturing, industrial production, electronic information, and other fields. However, while in use, the material surface is easily affected by dust and other pollutants so that its performance decreases. Therefore, it is necessary to develop superhydrophobic surfaces with self-cleaning and low adhesion properties. At present, this kind of composite functional surface is mainly realized by constructing micro-nano structures combined with low surface-energy chemical modifications. These processing methods greatly improve the hydrophobic performance of metal surfaces; however, they limit the working conditions because of the need for terminal low-energy modification conditions. Therefore, it is urgent to adopt simple and efficient methods for preparing high-performance composite metal surfaces. Laser processing methods have advantages such as a wide range of applicability, convenient operation, and non-contact implementation; moreover, they are highly adjustable and yield high processing accuracy for the micro-nano processing of materials. In this study, a biomimetic micron-scale structure was constructed on the surface of an aluminum alloy by a laser ablation method, and a superhydrophobic low viscosity self-cleaning surface was obtained by adjusting the laser processing parameters (Fig.1).
In this work, the unique structure of butterfly wings was used as the model for a biomimetic grid; the grid groove structure was prepared on the surface of a sandblasted, coarse-grained aluminum alloy by laser processing. The surface structure of the biomimetic metal sample was optimized by adjusting the laser processing spacing. The surface morphology, chemical composition, and surface wettability of the materials were characterized by scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), X-ray photoelectron spectroscopy (XPS), optical contact angle measurements, and high-speed imaging.
The results show that: (1) After coarsening by sandblasting, the surface of the specimen formed a micron-grade sheet structure. To obtain a controlled roughness, the primary structure was formed by a high-energy laser beam that transformed the sample surface into a complex grid structure composed of a coarse basement and grooves forming the secondary structure. With the increase of separation between the laser processing paths, metal spatter accumulated at the edge of the gridded groove, forming an integrated, interconnected, three-stage structure with staggered grooves (Fig.2). (2) The experimental contact angle measurement results show that the maximum static contact angle of the biomimetic sample surface reached 162.6° when the laser processing interval was 50 μm. The hydrophobicity of the biomimetic sample surface occurs because the microstructure formed by laser processing increases the gas-phase proportion of the solid surface. The dynamic contact angle test on the surface of the biomimetic sample shows that the best rolling angle (less than 5°) was achieved when the laser machining distance was 50 μm. The droplet slide test and droplet bounce test show that the sample has good low adhesion properties (Fig.3 and Table 1). (3) Two different pollutants, solid-phase dust and solid-liquid mixed mud, were selected for the self-cleaning tests of the sample surface, and the sample surface showed a good self-cleaning effect (Fig.4). (4) The samples were analyzed by XPS before and after the laser processing. Comparing these results shows that the number of nonpolar groups (i.e., C—C bonds) on the surface of the biomimetic sample increased compared to the polished sample. Since the nonpolar groups play an important role in improving the hydrophobicity, these results suggest that the compounds adsorbed from the air after laser processing are mainly nonpolar compounds. Therefore, the surface hydrophobicity of the biomimetic sample is greatly improved (Fig.5). (5) The mechanism of adhesion force was analyzed in detail by mechanical modeling, and it was found that the adhesion force was proportional to the contact area of the droplet-solid interface. When the laser processing interval was 50 μm, the contact area of the droplet-solid interface on the surface of the biomimetic sample was the smallest, and the adhesion force was the lowest (Fig.6).
In this study, a multi-layer grid structure similar to the scales of a butterfly wing was constructed on the surface of an aluminum alloy by laser ablation of a surface that had been previously roughened by sandblasting; a functional surface with superhydrophobicity, low adhesion, and self-cleaning properties was obtained. It appears that a surface with appropriate multistage structural characteristics can retain more air in close proximity to the surface of the aluminum alloy material surface; this "air effect" is responsible for the superhydrophobicity, low adhesion, and cleaning effect. The adhesion properties of the laser ablated biomimetic surface were studied by analyzing the bouncing behavior of droplets on different surfaces under the same conditions and by observing the sliding phenomenon as a function of inclination angle; the laser ablated surfaces have significantly lower adhesion than the polished surface. In addition, the experiment using dust and mud to simulate environmental pollution shows that when the biomimetic sample was placed at a 5° tilt angle, water droplets falling on the surface rolled along the tilt direction under the action of gravity without wetting the surface and removed the dust impurities attached to the surface. The sample was also placed into homemade mud, and the surface of the sample was found to be clean after removal, showing a very good non-wetting and self-cleaning effect. The mechanical analysis of the droplet on the surface of the inclined sample shows that, in the multistage structure with 50 μm separation between laser ablation lines, the droplet had the minimum adhesion force and could roll easily, thereby removing pollutants; this verifies that the sample has low adhesion and good self-cleaning performance. This biomimetic composite surface has the potential to contribute in the areas of desorption, drag reduction, and self-cleaning.
