# Superhydrophobic Surfaces Enabled by Femtosecond Fiber Laser-Written Nanostructures
## 1 Introduction  
Nature has created many living species with diverse wetting surfaces so that they can adapt to their habitats. Examples include self-cleaning lotus leaves [1] and rose petals [2], water-collecting Namib desert beetles [3], _Morpho_ butterflies with unidirectional-wetting wings [4], etc. In 1997, [PERSON] and [PERSON] revealed nature's mystery for the first time and found that lotus leaves are comprised of randomly oriented papillose epidermal cells covered with hydrophobic epicticular wax [5]. Their findings stirred great interest in hydrophobicity research [6], and since then this area has attracted increasing attention due to its wide range of applications in the automobile, aerospace, nautical, plumbing, and biomedical industries for functions such as hydrophobic surface corrosion resistance, rust prevention, friction reduction, and the improvement of cell adhesion [7; 8; 9; 10; 11; 12]. In the hydrophobic realm, automobiles and aircraft can have self-cleaning and anti-frosting properties, and medical implants can prevent blood coagulation and reduce the incidence of diseases like stent thrombosis [10; 11; 12]. Four methods of creating hydrophobic surfaces are adopted in this current field: chemical etching [13], reactive-ion etching [14], lithography [15; 16], and coating [17]. These processes alter the material surface to make it hydrophobic through reactions with acids and bases, the induction of plasma, exposure to radiation, or coatings involving chemicals. Although the processed surfaces can exhibit hydrophobic behavior, these techniques usually involve complicated fabrication processes and have many limitations, including the need for hazardous x-rays, chemicals, and multiple stepsthat reduce the yield. Moreover, they lack long-term durability and stability in terms of maintaining hydrophobic behavior and surface structures [18]. Laser irradiation using pulsed lasers has been widely accepted as one of the next generation technologies as it involves a relatively simple process, has a potentially low operation cost, and avoids hazardous materials. In addition, the at will feature of the laser enables it to create any complex pattern on any material and surface morphology for a myriad of applications. Through well-controlled melting, ablation, and re-solidification, the laser writes nanostructures on top of microstructures on the material surface, producing robust hydrophobic hierarchical structures [19]. Current research on laser-based hydrophobic processes mainly uses femtosecond (fs) solid state lasers (e.g., Ti:sapphire). A group of scientists have applied this method to their research [20; 21]. By utilizing the laser's powerful pulses to produce microgrooves on the metal surface, they controlled the wetting properties of the metal and created a new surface that was highly absorptive, superhydrophobic, and self-cleaning. Another group manufactured hierarchical structures with an fs laser [22]. They divided the process into two steps: first, a microscale ablation was produced on stainless steel at a high pulse energy and with different velocities; second, nanostructures were ablated on top of the micro-structures using laser-induced periodic surface structures (250 nm high and with a period of 600 nm) to create a hierarchical structure. The results of these groups are promising but the processes are slow. Currently, it takes an hour to process a \(1\times 1\) inch area [23]. This is because the solid state Ti:Sapphire fs lasers usually have a low pulse repetition rate (PRR) of less than 10 kHz [24]. Moreover, these solid state lasers are hard to maintain and operate. All of these characteristics hinder the potential industrial applications of hydrophobic surfaces produced by fs solid state lasers. In contrast, the fs fiber laser offers an emerging technology with unique features, such as its compact size (5\(\times\) smaller), low price, low maintenance, high PRR (hundreds of kHz versus 10 kHz), high wall plug efficiency (30% versus 1%), and better beam quality.