# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 1 Introduction
In conclusion, FS laser surface structuring is a simple, maskless, single-step, and non-contact process, enabling the controllable fabrication of various complex two-dimensional (2D) and three-dimensional (3D) structures [28, 29, 30, 31, 32, 33] with high precision, as presented in Figure 1.  
The interaction of FS laser pulses with matter involves several processes and phenomena, taking place on short timescales. During the first moments of interaction, instantaneous excitation of electrons occurs due to linear or non-linear photon absorption processes. The former takes place in metallic materials--contrary to semiconducting and dielectric ones--where non-linear absorption processes become more pronounced. As a result of photon absorption, the electron temperature increases to much higher values compared with that of the lattice, because electron-electron interaction is much faster than electron-phonon coupling. Subsequently, the hot electrons and cold lattice reach equilibrium in a few picoseconds through electron diffusion and electron-phonon interaction [34]. This two-temperature model is widely used to interpret the laser-matter dynamics on the electronic level, prior to the establishment of thermal equilibrium. With regard to laser-induced surface structures, the electron-phonon interaction is a key factor in understanding their formation. In particular, the energy transfer from the electrons to the lattice depends on the electron-phonon coupling constant; the higher this constant, e.g., in metals, the faster the electron-to-lattice energy transfer occurs [34]. Furthermore, the energy accumulation on the material's surface and, therefore, the hydrodynamics of the molten pool leading to laser-induced self-assembled structures under non-ablation conditions [35] are highly dependent on the electron-phonon coupling constant. Figure 2 shows the typical setup for FS laser-based surface structuring. The laser beam from the FS laser source is focused on the sample mounted on an XYZ translation stage controlled by a computer program. A combination of proper optics and lenses guides the laser beam to the sample. An electromechanical shutter placed in the beam's path interrupts the laser beam in raster scanning or spot-by-spot processing after a particular number of laser shots. The joulemeter is used to measure the energy deposited on the surface of the sample in real time. By controlling the laser fluence,  
Figure 1: Various two-dimensional (2D) and three-dimensional (3D) structures formed by femtosecond (FS) laser surface structuring under various ablation conditions [32]. Reproduced with permission from [32] line 1–3 Copyrights Royal Society of Chemistry 2015. Line 4 copyrights Elsevier 2014.  
wavelength, repetition rate, number of pulses, and pulse duration, various surface structures are formed on different materials. Over the last decade, FS laser-induced surface structures have been successfully used to control the wetting response of materials [12], especially for the fabrication of surfaces exhibiting extreme wetting properties, from superhydrophobic to superhydrophobic and from superoleophobic to superoleophilic. Due to their remarkable superwetting characteristics, FS laser surface structures have recently been employed for applications in oil-water separation. In this article, we will critically review the extreme wetting properties reported for the FS laser-induced surface structures, as well as the research progress on the production of oil-water separation membranes by means of FS laser micro/nano fabrication.