Fabrication of superwetting surfaces by ultrafast lasers and mechanical durability of superhydrophobic surfaces

With millions of years of evolution, nature developed various unique functional structures and intriguing phenomena, providing inexhaustible insights into design rules for biomimics. As one of those wonderful phenomena, superwetting surfaces, including superhydrophobic and superhydrophilic surfaces, have greatly excited researchers’ curiosity. By mimicking the natural superwetting structures like superhydrophobic lotus leaves, rose petals, butterfly wings, water strider legs etc. and superhydrophilic surfaces such as shark skin, nepenthes and many others, various artificial superwetting surfaces with advanced functions have been developed. It has been shown that the superhydrophobicity of a surface depends on both the low surface energy and the micro/nano scale hierarchical structures which trap air in their asperities. There are many methods to fabricate micro/nano structures, like templating, lithography, sol-gel, electrodeposition, spraying and others. But all these methods have their intrinsic limits, such as complex procedures, involving dangerous chemicals and dependence on material types. On the other hand, the development of ultrafast lasers provides a new powerful tool for micro/nano structures fabrication. As an alternative, ultrafast laser possesses a combination of excellent advantages, such as high efficiency, flexibility and controllability, non-contacting process with small heat-affected zone and high resolution, independence of material types and more, making it a great tool for superwetting micro/nano structures fabrication on solid surfaces. Although remarkable progress has been achieved on the study of superhydrophobic surfaces over the past 20 years, the commercialized practical applications are still very limited. One of the main reasons is attributed to the poor mechanical durability of the prepared superhydrophobic surfaces. Superhydrophobic surfaces are usually vulnerable and can be easily degraded by droplet impact, mechanical scratch, abrasion or vibration and so on, greatly constraining the long-term working performance of superhydrophobic surfaces, which are the hang-ups faced by the researches around the world. Hence, it is important and necessary to summarize the existing research and discuss the challenges faced in mechanical durability of superhydrophobic surfaces, and to review the ongoing research trends in the field of ultrafast laser superwetting surface fabrication. In this review, the recent advances of biomimetic superwetting surfaces fabricated by ultrafast lasers and the mechanical durability of superhydrophobic surfaces are summarized. First, the characteristics and methods of ultrafast laser fabrication are introduced. Then, some well-known natural superwetting surfaces are described. After this, different bio-inspired artificial superwetting surfaces and their potential applications are elaborated in a sequence as follows: Superhydrophobic surfaces, superhydrophobic surfaces with high transparency, superamphiphobic surfaces, superhydrophilic surfaces, hybrid superhydrophobic and superhydrophilic surfaces, smart superhydrophobic surfaces, self-healing superhydrophobic surfaces, slippery liquid-infused porous surfaces and superwetting surfaces with different adhesion and anisotropy. Subsequently, the mechanical durability of superhyrdrophobic surfaces and its testing and characterizing methods are summarized and discussed. In this section, the applicable conditions for each testing method and the methods to improve mechanical durability of superhydrophobic surfaces are discussed. In the end, the problems and challenges faced and the future development prospect in the field of ultrafast laser micro/nano fabrication are discussed. Ultrafast laser surface micro/nano fabrication is a very popular processing method. With the unknown secrets of nature being increasingly revealed and the joint efforts of all scientists and engineers, the research of bio-inspired superwetting surfaces will open a new chapter in modern technology.