The Role of the Surface Nano-Roughness on the Wettability Performance of Microstructured Metallic Surface Using Direct Laser Interference Patterning.

Bogdan Voisiat,Tim Kunze,Alfredo I. Aguilar-Morales,Andrés F. Lasagni,Sabri Alamri

doi:10.3390/ma12172737

Abstract

Superhydrophobic natural surfaces usually have multiple levels of structure hierarchy, particularly microstructures covered with nano-roughness. The multi-scale nature of such a surface reduces the wetting of water and oils, and supports self-cleaning properties. In this work, in order to broaden our understanding of the wetting properties of technical surfaces, biomimetic surface patterns were fabricated on stainless steel with single and multi-scale periodic structures using direct laser interference patterning (DLIP). Micropillars with a spatial period of 5.5 µm and a structural depth of 4.2 µm were fabricated and covered by a sub-micro roughness by using ultrashort laser pulses, thus obtaining a hierarchical geometry. In order to distinguish the influence of the different features on the wettability behavior, a nanosecond laser source was used to melt the nano-roughness, and thus to obtain single-scale patterns. Then, a systematic comparison between the single- and multi-scale structures was performed. Although, the treated surfaces showed hydrophilic behavior directly after the laser treatment, over time they reached a steady-state hydrophobic condition. However, the multi-scale structured metal showed a contact angle 31° higher than the single-scale geometry when the steady-state conditions were reached. Furthermore, the impact of the surface chemistry was investigated by energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analyses. Finally, a hydrophobizing agent was applied to the laser treated samples in order to further enhance the water contact angles and to determine the pure contribution of the surface topography. In the latter case, the multi-scale periodic microstructures reached static contact angles of 152° ± 2° and a contact angle hysteresis of only 4° ± 2°, while the single-scale structures did not show superhydrophobic behavior. These results definitely suggest that multi-scale DLIP structures in conjunction with a surface chemistry modification can promote a superhydrophobic regime.

Highlights

The enhancement of surface properties is currently addressed by using biomimetics since biological species represent optimized solutions to particular environmental conditions [1]
The typical morphology of the produced topographies can be seen produced. These small features can be identified as low spatial frequency laser induced periodic surface structures (LIPSS) (LSFL) with a spatial in
The LSFL are oriented parallel to the second line-like direct laser interference patterning (DLIP) treatment produced. These small features can be identified as low spatial frequency LIPSS (LSFL) with a spatial and are perpendicular to the laser beam polarization

Summary

Introduction

The enhancement of surface properties is currently addressed by using biomimetics since biological species represent optimized solutions to particular environmental conditions [1]. Superhydrophobic behavior, that is, the capacity to repel water, is typically achieved if the static water contact angle (SWCA) exceeds 150◦. The main association between surface roughness, chemistry and water-repellency has already been explained by Wenzel [9], as well as by Cassie and Baxter [10]. The Wenzel model describes a wetting regime in which the liquid penetrates into the roughened surface (complete contact between liquid and solid surface is always present), whereas the Cassie-Baxter model outlines the importance of trapped air between the solid surface and liquid [11]. For surfaces exhibiting a Wenzel behavior, the CAH is typically high while in the case of the Cassie-Baxter condition it assumes low values. Superhydrophobic surfaces should have a SWCA above 150◦ as well as CAHs below 10◦ [12]

Methods

Results

Conclusion