# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 6 Summary and Outlook  
In summary, FS laser surface structuring is a new technology that can be used to create micro- and nanostructures on virtually any type of material. As a consequence of its remarkable ability to control the morphological features at multiple length scales, FS laser processing is a unique tool to tune the wettability of solid surfaces. This capability demonstrates the great potential of FS structuring techniques for oil-water separation applications. Indeed, various approaches to control wettability using FS laser-fabricated surface micro/nanostructure have been explored recently. As a result, superwetting responses, including superhydrophobic-superoleophilic, superhydrophobic-underwater superoleophobic, underwater superoleophobic, and underoil superhydrophobic responses, have been demonstrated for such surfaces and have shown promising results with respect to oil-water separation. This approach has some advantages compared with competitive oil-water separation techniques as far as the durability of the fabricated oil-water separation systems. Specifically, alternative technologies usually employ fragile and low-adhesion fluorinated compounds that lose their superwetting characteristic over a short period of time. On the contrary, FS laser-induced surface structuring can provide surface structures in a single step, without the need for additional chemical coatings. As a result, FS laser-structured filters/membranes have shown promising results of high efficiency, recyclability, and durability with respect to oil-water separation. Oil-water separation by FS laser surface structuring has been realized in two ways, namely meshes decorated with self-organized structures and arrays of microholes created by laser ablation. In both cases, surface structuring may also be followed by a chemical functionalization step. Both meshes and micro-through holes filters have been used for the separation of heavy and light oil/water mixtures. Although, FS laser surface structures display exceptional oil-water separation efficiency on a laboratory scale, their potential use for separating large volumes of oil-water mixtures in oil spill accidents and organic wastes in industries requires further research. It should be considered that in  
Figure 14: (**a–d**) SEM image of a FS laser-fabricated titanium membrane filter showing the presence of microprotrusions at the periphery of the microholes; (**e,f**) EDX analysis of the pristine and FS laser-treated titanium foil [69]. most of the separation studies, self-prepared oil-water mixtures have been used, which do not perfectly simulate the properties of real oil-water mixtures present in oil leakages and spills. Indeed, oil spills and organic industrial wastes consist of an amalgam of hydrocarbons of different densities, including thick crude oil, emulsions, and surfactants, which could cause fouling and blockage of the micropores present in separation systems. In this context, studies addressing the separation of emulsions and impure organic compounds via FS laser-structured separation systems should be performed. Another study that requires special attention is that of separation at high flux rates, mostly related to the systems used for continuous separation of oil-water mixtures. Finally, it should be emphasized that the use of FS laser-generated self-organized surface structures for oil-water separation is still considerably limited. To date, only LISR have been used; however, this is not the case for membranes decorated with LISG and LISS structures. Moreover, the use of FS high-pulse energies in laser micromachining via ablation has not been explored yet. The surface structures formed as a result of high-pulse energy laser ablation are expected to provide exceptional results. These studies will also be useful in adapting this technology to large-area membrane systems fabricated in a short period of time.