Nature-inspired superwettability achieved by femtosecond lasers

Over billions of years of evolution, many organisms in nature have exhibited unique surface wettability. For example, lotus leaves have the self-cleaning function. Water striders can walk on water. Butterflies can fly in the rain. Mosquito eyes can prevent fog. Fish cannot be contaminated by oil underwater. Desert beetles, cacti, and spider silk can collect water mist. Inspired by these extreme wetting phenomena in nature, a series of super wettabilities has been achieved based on different micro/ nano-machining methods because the wettability of the material surface mainly depends on the chemical composition and the microstructure of the solid surface. Although the most traditional micro/nano-processing methods can be used to achieve  superwettability, these methods are more or less subject to certain inherent limitations, such as complex preparation steps, limited to specific materials, and lack of flexibility. It is still a great challenge to develop a versatile and simple tool/method to produce various superwetting microstructures on any solid substrates.

The laser is one of the greatest inventions of the 20th century. Femtosecond (10^-15 s) laser, as an advanced ultrashort pulsed laser, has two features of ultra-short pulse width and extremely high peak intensity. In recent years, the femtosecond laser has become an essential tool for modern extreme and ultra-precision manufacturing. The femtosecond laser can ablate almost any material, resulting in microstructures on the material’s surface. Femtosecond laser also has advantages in the fine design of micro/nanostructure. Because the surface microstructure has a significant effect on the wettability of solid substrates, femtosecond laser processing is used to form special microscale and nanoscale structures on substrate surfaces and obtain various superwettabilities. Femtosecond laser shows a powerful ability to design and modify the wettability of materials.

Recently, Yong et al. systematically summarizes the realizations and applications of the superwettability achieved by femtosecond laser. Inspired by nature, superwetting microstructure can be easily prepared on the surface of solid materials by femtosecond laser processing. Superhydrophilicity can be obtained by forming a hierarchical microstructure on a hydrophilic substrate, which causes water to thoroughly wet the material surface. Superhydrophobicity is a synergistic effect between surface microstructure and low-surface-energy chemistry. To achieve superhydrophobicity, it is necessary to use a femtosecond laser to directly construct the hierarchical microstructures on the hydrophobic substrate or build microstructure on the hydrophilic substrate followed by low-surface-energy modification. Superhydrophobic materials greatly repel aqueous solutions. The re-entrant microstructure is the key to realizing superamphiphobicity. The superamphiphobic surfaces also have superoleophobicity as well as superhydrophobicity. Unlike the preparation of superamphiphobic surfaces, underwater superoleophobic surfaces can be easily fabricated by generating microstructure on a hydrophilic substrate. Both types of superoleophobic surfaces have a strong repellence to oils and oily liquids, but superamphiphobic materials work in the air while underwater superoleophobic materials work in the water. Femtosecond laser-induced superhydrophilic microstructures usually show superaerophobicity, and the superhydrophobic microstructures show superaerophilicity in a water medium. The underwater superaerophobic surfaces can repel bubbles so that bubbles are difficult to adhere to the material surface in water. In contrast, bubbles can easily spread over underwater superaerophilic surfaces and be absorbed by solid surfaces. The femtosecond laser can directly write porous microstructures on many solid substrates. The lubricating fluid is further filled into the laser-induced pores to form a thin lubricating layer on the surface of the solid substrate. The as-prepared slippery surfaces show excellent repellence against various pure or composite liquids. The emerging superpolymphobicity and supermetalphobicity are also briefly introduced. The laser-induced microstructures that repel underwater liquid polymers or liquid metals can be used to manipulate and pattern polymers and liquid metals. The unique surface wettability enables the femtosecond laser-processed materials to have rich practical applications in anti-liquids, self-cleaning, anti-icing/fogging/snowing, antifouling, oil/water separation, anticorrosion, drag reduction, water harvesting, manipulation of liquid droplets, liquid patterning, microfluidics, lab chip, cell engineering, buoyancy enhancement, and so on.

Although the technology of femtosecond laser controlling surface wettability is still limited by the defect of low machining efficiency, its strong flexibility and the ability to process most given materials are irreplaceable by other micromanufacturing technologies. With more superwetting surfaces being designed and prepared and new applications emerging, this research direction will have a promising future. We believe that the large-scale, commercial application of the femtosecond laser-induced superwetting materials is not far off.