Elsevier

Surface and Coatings Technology

Volume 363, 15 April 2019, Pages 170-178
Surface and Coatings Technology

Achieving of bionic super-hydrophobicity by electrodepositing nano-Ni-pyramids on the picosecond laser-ablated micro-Cu-cone surface

https://doi.org/10.1016/j.surfcoat.2019.02.037Get rights and content

Highlights

  • Electrodepositing dense nano-Ni-pyramids on the laser-ablated micro-cones.

  • Sequentially processed surface is associated with better Cassie-state stability.

  • The stable Cassie state is attributed to the dense nano-Ni-pyramids.

  • The sequentially processed surfaces possess long-term durability.

  • The sequential processing technique is applied on the inclined surfaces.

Abstract

The fabrication of a bionic super-hydrophobic metallic surface without any low-surface-energy modification is investigated, in which picosecond laser ablation, electropolishing and electrodeposition are used sequentially. The micro-cone structure is formed via laser ablation, and then the nano-pyramid structure is electrodeposited on the micro-cone structure, resulting in a hierarchical structure that is important for achieving super-hydrophobicity on an intrinsically hydrophilic material surface. In this study, the surface morphology and material removal mechanism are discussed with respect to the laser-ablated surface and the subsequent electropolished/electrodeposited surfaces. The condensation experiments results show that both types of surfaces are associated with relatively high contact angles (CAs) at normal temperatures, and the CAs are 147° and 160°, respectively. However, the CA on the laser-ablated surfaces at low temperatures decreases to as less as 107°, which may be owing to the easy transition from Cassie state to a metastable state or even the Wenzel state, leading to a degraded hydrophobic surface. By contrast, the CA on the sequentially processed surfaces still maintains at 150°, which should be attributed to the densely distributed nano-scale structures. The sequentially processed surfaces also show excellent long-term durability. Furthermore, this technique has been employed for fabricating super-hydrophobic surfaces with CAs > 151° and the sliding angles (SAs) < 8° on the inclined surfaces with the inclination angle α < 30°. Overall, the technique presented in this study supply a practical and reliable method for realizing the stable Cassie state and hence super-hydrophobicity on metallic surfaces.

Introduction

In the nature, super-hydrophobic surfaces can be found in various animals and plants, including leeches, lotus and the aquatic fern Salvinia molesta [[1], [2], [3], [4]]. Water droplets roll easily on these surfaces as the contact angles (CAs) exceed 150° and the corresponding sliding angles (SAs) are <10°, which yields unique properties such as being water-repellent [5], self-cleaning [[6], [7], [8]] and anti-icing [9]. Therefore, bionic methods for processing super-hydrophobic surfaces have attracted much attention, among which picosecond laser texturing of metal surfaces has drawn an enormous amount of research interest due to its advantages of high precision, limited thermal damage, controllability and low pollution [10,11]. However, roughening a metal surface with a laser makes it more hydrophilic because metals are intrinsically hydrophilic materials and have high-surface-energy. To reduce the surface energy after laser irradiation, a layer of low-surface-energy material is usually introduced via chemical modification [[12], [13], [14]]. However, chemically modified layers are usually associated with poor thermal and mechanical stability and are easily damaged in harsh environments. In addition, they affect the inherent properties of metals, such as the surface conductivity and reflectivity [15]. Therefore, techniques are desired to introduce hydrophobic surfaces without the need of chemical modification.

In the past few years, many studies were conducted on processing metals without chemical modification. Bhattacharya et al. [16] were the first to report roughness-induced hydrophobicity of a metallic surface with hydrophobic clustered Cu nano-wires, in which low-surface-energy materials were not involved. Specifically, a ‘two-scale’ roughness surface was made by depositing Cu-nano-wire arrays through porous anodic Al and the following vacuum-dried step. In addition, Kwon et al. [17] used nano-second laser ablation (LA) combined with insulation and electrodeposition to fabricate a micro-pillar array with a re-entrant structure of Cu on stainless steel, for which the water CA was up to 153°. Recently, Yang et al. [18] prepared the superhydrophobic surface with the CA of 153° and the SA of 5.5° on the Al surface without chemical modification via nanosecond laser ablation. They also systematically investigated the wettability transition mechanism by analyzing surface morphology and surface chemical compositions. Although some papers regarding different methods used for fabricating metallic superhydrophobic surfaces without chemical modification, a practical and efficient processing method for mechanical stable super-hydrophobic surface with Cassie-state stability at low temperature and suitable for most metals is relatively limited.

Herein, we fabricated biomimetic super-hydrophobic metal surfaces using different methods, i.e. LA and a sequential processing (LA, electropolishing and electrodeposition). The mechanism for micro-nano hierarchical structure formation was analysed and the condensation experiments were conducted to compare the hydrophobicity and Cassie-state stability of the LA and sequentially processed surfaces. After the analysis of the condensation mechanism, the durability of the sequentially processed surface was analysed by aging experiments. In addition, the sequential processing technique was applied on the inclined surfaces on which we successfully fabricated a super-hydrophobic layer containing micro-nano hierarchical structures.

Section snippets

Experimental details

The experimental device to prepare the hierarchical micro-nano structures comprised an ultra-fast picosecond laser system for the micro-structures and an electropolishing and electrodeposition system for the nano-structures, as shown in Fig. 1(a). A schematic of the overall sequential processing is shown in Fig. 1(b), including arranging the scanning path, LA, electropolishing and electrodeposition. The sample was a 99.9 wt% Cu plate (15 × 15 × 1 mm3). Before the picosecond LA, the sample was

Surface morphology

To compare the surface morphologies of the micro-nano hierarchical structures progressively formed by the techniques used in this study, three types of structures were prepared according to the processing sequence, namely Structure-1 (LA), Structure-2 (LA and electropolishing) and Structure-3 (LA, electropolishing and electrodeposition).

The sample with the periodic micro-nano structures shown in the top row of Fig. 2 (i.e. Structure-1) was fabricated by LA only. The period of these micro-cones

Conclusion

A sequential fabrication technology has been proposed for bionic super-hydrophobic metallic surfaces via picosecond LA, electropolishing and electrodeposition. Using this technology, super-hydrophobic surfaces with micro-nano hierarchical structures were prepared successfully. The most significant advantages and conclusions of sequential processing technique can be summarized as following:

  • (1)

    Compared with a surface prepared by LA, a sequentially processed surface is covered with densely

Acknowledgement

The authors would like to thank the support of National Natural Science Foundation of China (No. 51675242; No. 11504144), Six Talent Peaks Project in Jiangsu Province (No. GDZB-019), Natural Science Research Project for Universities in Jiangsu Province (18KJB460005) and National Key Laboratory of Science and Technology on Vacuum Technology and Physics (No. 614220701050817).

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