Recent progress in femtosecond laser-induced superwetting surfaces

The materials with various superwettabilities have attracted increasing interests because of their broad applications, such as water/oil-repellent coating, self-cleaning coating, anti-ice/fog/snow, manipulation of droplets, oil/water separation, antifouling, anti-corrosion, underwater drag reduction, lab chip, cell engineering, fog collection, microfluidic systems, and so on. After millions of years of evolution, natural organisms have evolved perfect structures and multifunctional surfaces. The surfaces of many plants and animals are endowed with special wettability, which inspire scientists to design and fabricate bionic surfaces with superwettability. It is found that the surface wettability is mainly determined by the chemical composition and surface microstructure. Inspired by nature (e.g., lotus leaf, fish scale, pitcher plant, red rose petal, rice leaf, butterfly wings), a large number of superwetting surfaces have been developed by various microfabrication technologies. However, those methods, more or less, are suffered from some inherent limitations, e.g., tight restriction on special materials, lack flexibility, and complex fabrication process. The development of a widely applicable and simple tool that can easily achieve various superwettabilities is still the major trend in this research field. Femtosecond laser has been proven to be a strong tool in advanced micro/nanofabrication. Recently, this tool is also successfully applied to design and change the surface wettability of different solid substrates. The femtosecond laser microfabrication has many advantages, such as small heat-affected zone, precise ablation threshold, non-contact process, and high resolution. In addition, the femtosecond laser can ablate almost all of the materials, including semiconductors, metals, polymers, glasses, and ceramics. By the simple one-step scanning manner, micro/nanoscale hierarchical structures can be directly created on the surfaces of different materials via the femtosecond laser treatment. Taking advantages of these features, femtosecond laser microfabrication have also been a great success in preparing superwetting surfaces. In this review, we summarize the recent developments in femtosecond laser-structured surfaces with special wettability. The article starts with the introduction related to the theoretical basis of wettability and the features of femtosecond laser microfabrication, as the background. Then, different superwettabilities achieved by femtosecond laser treatment are summarized, grouped by the characteristic of wettability. Inspired by nature, those laser-induced surface microstructures have superhydrophobicity, underwater superoleophobicity, underwater superaerophobicity, slippery liquid-infused porous surface, controllable adhesion, and anisotropic wettability, respectively. In each class of properties, the biological surface with such superwettability is introduced in advance. Then, how to use a femtosecond laser to design and fabricate these special superwetting microstructures was discussed, i.e., the preparation principle. According to the formation mechanism of these superwetting states, the internal relation between the femtosecond laser-induced surface microstructure and the macroscopic wettability is revealed; that is, surface microstructure determines the surface wettability. Next, some examples about the practical applications of the femtosecond laser-designed superwetting surfaces are also introduced following various superwettabilities, such as self-cleaning, manipulation of liquid droplets, oil/water separation, cell engineering, fog harvesting, buoyancy enhancement, liquid patterning, underwater gas capture, and detection analysis. Finally, the existing challenges and future prospects of this fast-growing field are discussed. The technology of femtosecond laser-controlled surface wettability is still irreplaceable by other microfabrication methods because of its special advantages. Such technology is better at designing and preparing fine microstructures and complex and heterogeneous wettability. This technology will have superiority in some special applications, such as cell engineering, military facilities, etc. With more and more scientists and engineers getting into this research field and being devoted to fabricating various superwetting surfaces, this research direction will have a bright and exciting future.