Effect of sub-millimetre morphologies on the hydrophobicity of a copper surface prepared by WEDM

Abstract

To study the influence of the geometric structure on the surface wettability of metal materials, sub-millimetre sinusoidal and rectangular shapes were prepared on the surface of a Cu alloy by wire electrical discharge machining (WEDM). It was found that micro- and nano-scale craters and asperities are formed on the surface of the Cu alloy due to the electric spark discharge process, which leads to a multi-scale hierarchical surfaces formation on the Cu alloys. In addition, the thermal action of the spark discharge causes rapid remelting on the surface of the Cu alloy to form a recast layer with an elevated hardness but does not significantly affect the chemical composition. The formation of multi-scale sub-millimetre structures improves the hydrophobicity of the surface of the Cu alloy, wherein the maximum contact angles of the sinusoidal structure and the rectangular structure can reach 152.1° and 149.1°, respectively. Periodic changes in the structure effect on the wettability of the material surface. For the sinusoidal structures, the static contact angle of the droplets is not sensitive to periodic changes when the period is small, but when the period increases to a certain value, the wetting state of the droplets changes and the contact angle decreases significantly. For the rectangular structure, the contact angle of the droplets first increases and then decreases with increasing period. In contrast, the sinusoidal structures exhibit a better hydrophobicity than that of the rectangular structures due to changes in the curvature of the solid-liquid contact surface. The servo voltage during WEDM can significantly affect the shape and size of the micron-scale structures on the surface, resulting in the static contact angle of the droplet increasing first and then decreasing with an increase in the servo voltage.