# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 5 Oil-Water Separation via FS Laser Structuring
### FS Laser-Structured Meshes  
[PERSON] et al. reported on a Janus wire mesh for oil-water separation (Figure 7) that exhibited different wettability on its opposite sides [66]. One side was superhydrophobic-superoleophilic, while the other side was superhydrophobic-underwater superoleophobic. Initially, LISR were formed on both sides of the copper mesh, and then, the superhydrophobic side was prepared by surface functionalization with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. Similarly, the underwater superoleophobic side was formed via dipping into a graphene oxide aqueous solution. Figure 7a-h shows SEM images and the elemental map of both sides of the mesh, which confirm the modification with fluorosilane and graphene oxide, respectively. It was observed that the opposite wetting responses present on the same mesh facilitate the separation of light and heavy oils from water. For light oil separation, the superhydrophobic-superoleophilic side was brought in contact with the oil-water mixture, whereas for heavy oil separation, the oil-water mixture was placed on the superhydrophobic-underwater superoleophobic side. As a result, for both heavy and light oil-water mixtures, a separation efficiency greater than 99% was achieved. [PERSON] et al. used a FS laser to generate LISR on stainless-steel mesh for oil-water separation purposes [67]. Figure 8a,b shows an optical image of the mesh before and after laser treatment, respectively; following the laser treatment, the mesh color turned to black. Figure 8c-e shows that the uniform nanoripples, exhibiting a 500-800 nm period and an average depth of 130 nm, are covered with nanoparticles of variable sizes, formed upon FS laser treatment. Figure 8f,g presents the chemical composition of the pristine and laser-treated meshes. The latter showed an increase in oxygen content, attributed to the oxidation of Cr, Mn, and Fe present in the stainless steel. As a consequence of the laser treatment, the mesh wettability changed from having weak hydrophobicity in its pristine state to being superhydrophilic. Moreover, the underwater contact angle of a 1,2-dichloro oil droplet on the laser-processed mesh was equal to 157.2\({}^{\circ}\), and the sliding angle was 5\({}^{\circ}\). An oil-water separation process using laser-structured mesh is demonstrated in Figure 9a-d. An oil-water mixture was poured onto the pre-wetted laser-treated mesh positioned above two beakers at a \(\sim\)25\({}^{\circ}\) tilt angle. It was observed that water passed through the mesh and dripped off into the first beaker, while oil was repelled and rolled off the mesh surface and was collected in a separate container. Laser-treated stainless-steel mesh was tested with different oil-water mixtures, and all showed high OACs \(>\)150\({}^{\circ}\), irrespective of oil type, and a high separation efficiency of 99%. The laser-processed mesh was quite durable and chemically inert, considering that it retained its wetting characteristics even when it was rubbed repeatedly with sand paper and after being treated with 1 M HCl, 1 M NaOH, and 10% NaCl solutions. Such severe treatments validate the use of laser-structured mesh in harsh environments. Figure 7: (**a,b**) SEM image of FS laser-structured Janus wire mesh. (**c**) Laser-structured fluorosilane-modified mesh (580 mW, 7.39 \(\times\) 106 mJ/cm\({}^{2}\)) at different magnifications. LISR with a period of 900 nm at fluorosilane-modified mesh. (**d**) Elemental map of fluorosilane-modified mesh showing the presence of fluorine and silicon. (**e,f**) Laser-structured GO-modified mesh (100 mW, 12.74 \(\times\) 105 mJ/cm\({}^{2}\)) at different magnifications. (**g**) LISR covered with GO wrinkles. (**h**) Elemental map of GO-modified mesh showing the high presence of oxygen and carbon [66]. Reproduced with permission from [66] Royal Society of Chemistry 2017. Figure 8: (**a**) Optical and (**b,c**) SEM images of stainless-steel mesh before and after FS laser treatment.