Objective Silicone rubber has been widely used in aerospace and power transportation because it has stable and reliable physical properties. However, the hydrophobicity of its surface should be improved to enhance its stability in practical applications. This property can be improved more quickly and effectively by laser etching than by surface coating, plasma processing, imprinting, and other methods. Various surface microstructures can also be obtained through laser etching. The main factor that causes the change in hydrophobicity is the rough microstructure on silicone rubber surfaces after laser irradiation. However, the specific influence of its surface morphology on hydrophobicity has not yet been confirmed. Fractal dimension is a measure to characterize the irregularity of complex shapes, which can indicate the effectiveness of the space occupied by complex shapes, and has been widely used in studies on the physical properties of rough surfaces. Therefore, in this study, fractal theory and fractal dimension are introduced to explore the rough structure and geometric characteristics of silicone rubber surfaces after laser etching, establish their association with surface hydrophobicity, and provide a method for explaining the change in the wettability of rough surfaces.
Methods A silicone rubber surface was etched with an SPI nanosecond fiber laser at a maximum power of 70 W and a wavelength of 1064 nm. Silicone rubber surfaces in different wetting states were obtained by modifying the laser fluence. The wettability of the surfaces was characterized by measuring their contact and rolling angles. Fourier transform attenuated total reflection infrared spectroscopy(ART-FTIR) and energy dispersive spectromete(EDS) were then conducted to detect the chemical elements and groups on the sample surfaces, and the influence of chemical factors on surface wettability was excluded. After the rough surface microstructure was determined as the main cause of the change in wettability, the contour curves of the sample surfaces collected with a white light interferometer (BRUKE, ContourGT-K0) were drawn to calculate the fractal dimension. Combined with the scanning electron microscope(SEM) micrograph of the sample surfaces, fractal theory was introduced to analyze the micro-nanocomposite structures produced on the laser-etched silicone rubber surface.
Results and Discussions Laser treatment could significantly improve the hydrophobicity of the silicone rubber surface. The surface of the untreated silicone rubber exhibited a weak hydrophobicity with contact and rolling angles of ~110° and >90°, respectively. As the laser energy input increased, the contact angle of the silicone rubber surface increased rapidly. When the laser fluence increased to 10 J/cm 2, the contact angle increased to ~160°, whereas the rolling angle decreased to ~3°. ART-FTIR and EDS revealed that the input laser energy did not induce the changes in the chemical elements and groups on the silicone rubber surfaces. The surface wettability of the laser-treated silicone rubber was mainly determined by its three-dimensional microstructure. The silicone rubber surface was pyrolyzed locally when the laser fluence was low. Consequently, a coarse structure with a high self-similarity and a composite state of large and small particles formed, thereby improving the fractal dimension of the surface and slightly increasing the surface hydrophobicity. As the laser fluence increased, the large particles on the silicone rubber were pyrolyzed to the micro-nanoparticles, which reduced the fractal dimension of the silicone rubber surface. Droplets were only in contact with the convex surface of the small particles on the surface, creating a superhydrophobic surface. As the laser fluence further increased, a plate-like structure with trenches was produced because of thermal effects, and the roughness of the processed surface increased. When the balance between the inputted laser energy and the surface pyrolysis of silicone rubber was reached, the rough structure of the surface no longer changed significantly. As a result, a stable superhydrophobic surface with a high self-similarity was created.
Conclusions When silicon rubber is etched with a nanosecond laser, the chemical element composition and groups on the surface do not vary significantly, and wettability changes mainly because of the surface microstructure. Therefore, the fractal characteristics of the rough structure of the laser-treated silicone rubber surface are analyzed to establish the relationship between surface microstructure characteristics and hydrophobicity. As the laser fluence increases, the highest fractal dimension of 1.65 is obtained when the silicone rubber surface is irradiated with a laser fluence of 7.5 J/cm 2. A micro-nanocomposite structure with a high self-similarity simultaneously appears on the surface of the silicone rubber, thereby improving its hydrophobic properties. When the laser fluence further increases to 10 J/cm 2, the large particles on the silicone rubber surface become refined into small particles and disperse on the surface. Consequently, the surface roughness of the silicone rubber and the fractal dimension decrease to 4--5 μm and 1.40, respectively. As a result, the contact state between the silicone rubber surface and the water droplets transforms from a Wenzel model to a Cassie model. In other words, the processed surface changes from a hydrophobic state to a superhydrophobic state. When the laser energy fluence further increases, the fractal dimension increases again and stabilizes at about 1.55. When the silicone rubber is irradiated with larger laser energy, small micro-nano particles continue to be generated on the surface. These small micro-nano particles are continuously stacked on the basis of the original particles, thereby forming a composite structure with a high self-similarity again. However, when the balance between the rate of the thermal cracking of the large particles and the formation of the small micro-nano particles is obtained, the self-similarity of the surface micro-nano structure no longer changes, and the surface hydrophobicity remains stable. Therefore, the analysis of the fractal characteristics of the micro-nano structure on the silicone rubber surface after laser etching helps establish the relationship between surface structure and hydrophobicity. It also provides a basis for rapidly preparing superhydrophobic silicone rubber surfaces and regulating their surface microstructure.
