Nanosecond laser pulse surface texturing for generation of superhydrophobic surfaces: Impact of laser and external process parameters

Superhydrophobic surfaces, which mimic the anti-wetting capabilities of the lotus leaf (Nelumbo nucifera) and similar naturally-occurring surfaces have been subject of significant interest in the engineering field for many years; in particular to create self-cleaning and anti-bacterial surfaces. A droplet of water landing on a superhydrophobic surface will attach itself to foreign debris and slide away. There have been a significant number of reports of ultra-short pulsed lasers being used to generate such surfaces, and rather fewer reports that use much lower cost nanosecond systems, which is the approach we have chosen to follow, in order to develop a low-cost process suitable for large surface areas. Flat sheets of the commonly used stainless steels 304 and 316 were textured using a nanosecond pulsed fibre laser operating at 1064nm, generating surface grooves of typical dimension 10-30 µm deep by 30 µm wide, dependent on the fluence. Quantitative analysis of the wettability of the laser micro-structured surfaces was carried by measuring the static contact angle of a droplet of deionized water with a volume in the microliter range. The transition between hydrophilic to hydrophobic was periodically monitored. Such laser structured metal surfaces are hydrophilic immediately after processing, transitioning to super hydrophobic in some cases, displaying static contact angles larger than 130°, however this can take up to several months. Our work has concentrated on speeding up this transition and to this end we have studied the impact of the laser processing and environmental parameters, and we have demonstrated that these can have significant impact on the wettability transition.