Preparation of hydrophobic surface on steel by patterning using laser ablation process
Student thesis: Master's Thesis
Currently there is an increasing demand for the application of hydrophobic surface in industrial and biological processes. It has been found that the contact angle with liquid, closely related to hydrophobicity of a solid surface, is largely determined by micro-geometrical structure and chemistry of the said surface. In this investigation, the hydrophobicity of steel surface was achieved by implementing micro-patterns on substrate using laser ablation process. For the patterning, a short pulse excimer laser with a wavelength of 248nm was used to etch the substrate surface to form different controlled patterns. Based on the models proposed by Wenzel and Cassie-Baxter, the depth of etched grooves and the clearance between two grooves were optimized. The water contact angle of the patterned surface was increased to about 130°, compared to 68.5° found from a plain smooth steel surface (Ra ≤ 0.01 μm). As a preliminary investigation, an amorphous carbon (a-C) coating was deposited on the patterned surface. It shows that the contact angle was increased further by about 10°-20° on a patterned surface. At the same time, the deformation in local surface induced by laser heating was considered. On the patterned surface, the melted substrate was observed to have formed slopes and piled to be ridges along the sides of grooves. With an assumption of overhangs which could suspend the free liquid surface in indentations on a hydrophilic substrate, the relationship between the contact angle and pattern parameters was also discussed. The different geometrical models of the surface profiles were employed to approximate the real laser patterned surface to forecast the contact angle with better accuracy. The real contact area between solid and liquid was calculated. The effect of ridges was also considered when the clearance between two grooves was smaller than 50μm. The original Cassie-Baxter equation was modified with appropriate geometrical models to calculate the contact angle successfully.
- Stainless steel, Hydrophobic surfaces, Laser ablation