# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 1 Introduction  
Surface structuring using femtosecond (FS) pulses gives rise to spatially resolved micro/nanometer-sized features that are attributed to the reduced thermal and mechanical effects upon the ultrafast laser-material interaction [1; 2]. It is a novel technique for precise control of a wide range of properties on virtually any type of material, including optical, mechanical, chemical, wetting, biological, and tribological properties [3]. As a result, FS laser-induced surface structures are used in a broad range of applications, including plasmonics, photonics, optoelectronics, photovoltaics, biomedicine, microfluidics, self-cleaning, and oil-water separation. For example, highly absorptive, black metallic [4], and semiconducting [5] surfaces formed by FS laser structuring are used in photovoltaics [6], as wellas in stealth technology [7], to entirely absorb ultraviolet -visible (UV-Vis) and infrared (IR) radiation. In addition, FS laser-fabricated, pigment-free, colored metals [8,9] and iridescent surfaces [10] can be applied for the fine-tuning of surface optical properties. Such colors are due to the plasmonic absorption at specific wavelengths featured by the laser-induced surface structures. Moreover, FS laser surface structuring can significantly alter the wetting properties of different materials. Inspired by the lotus leaf found in nature, superhydrophobic surfaces have been generated for applications in self-cleaning, anti-icing, drag reduction, corrosion resistance, and water harvesting [11, 12, 13]. In contrast to superhydrophobic surfaces, FS laser structuring can also provide, under certain irradiation conditions, surfaces capable of engulfing significant amounts of liquid, called super-wicking surfaces [14,15]. This effect is so strong that liquid could be transported uphill, against the gravitational pull, onto the structured surface. Recently, wettability control by varying the number of pulses and laser fluence in titanium [16] and copper [17], respectively, has been achieved by FS laser processing. It has been found that the number of incident pulses, pulse energy, laser fluence, and environment are important factors in realizing different levels of wettability [18,19]. Moreover, superhydrophobic surfaces recently fabricated by FS laser are able to repel complex liquid droplets, such as glucose, cola, juice, saline, and bovine serum albumin in water [20]. Other applications of FS laser-induced surface processing include the structuring of tissue engineering scaffolds showing enhanced cell adhesion properties [21, 22, 23], signal intensification for surface-enhanced Raman spectroscopy [24], enhancement of X-ray generation and photoelectron emission and manipulation of tribological properties [25,26]. The superior precision and accuracy of FS laser-induced surface structuring is a direct consequence of the shorter pulse duration and, thus, the laser-material interaction time [27]. In the case of longer pulses, the thermal energy generated upon melting diffuses around the irradiation spot, giving rise to the creation of heat-affected zones (HAZs). Above the material damage threshold, the HAZ has a detrimental effect on the integrity of the material by producing microcracks together with micropores. Besides this, when using longer pulses, the ablated material causes recast layers and surface debris, resulting in poor fabrication resolution. All such effects are avoided upon using FS laser pulses, due to the faster ablation process, i.e., rapid vaporization and material removal. Furthermore, FS laser surface structuring has various advantages compared with other techniques, such as photolithography, electron or ion beam; andmechanical and chemical methods. Such techniques involve multiple steps and tedious sample preparation and require vacuum conditions and hazardous chemicals.