2024 年 42 巻 p. 103-111
Shedding snow from highway structures, typically made of galvanized steel, can disrupt traffic, sometimes causing an accident. Here we examine the snow repellency of the galvanized steels —Zn–Al–Mg (ZAM) and Zn–Al (ZA)—whose surface was treated with a femtosecond laser. The treatment involves a laser fluence of 0.7 J/cm2 at a scanning speed of 2.0 mm/s. It produces surface structures with both nano-ripples and a non-periodic distribution of sub-micron spacing covered by a fine, ripple-like, periodic nanostructure. The resulting contact angle (CA) of water droplets on the surface was found to be about 147.0° on the laser-treated ZAM surface and 131.6° on the laser-treated ZA surface. However, at two months of outdoor exposure in winter in Hokkaido, the contact angles had decreased, and in some cases, the surfaces had become hydrophilic (averaged CA＝21.8°). Then, during snowfall and with the surface near 0 °C, snow was shed from the hydrophilic laser-treated ZAM and ZA samples, but snow remained on the non-lased ZAM sample. We argue that the hydrophilicity of the laser-treated surfaces facilitates the evaporation of surface-warmed meltwater due to the greater melt–air interface area. Moreover, the treatment darkens the surface, thus promoting melting via warming due to greater solar radiation absorption. The hydrophilic surfaces of laser-treated ZAM and ZA were found to be effective at melting snow near 0 °C, but snow was found to suddenly accrete on both surfaces when the temperature fell to about −11 °C.
