# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 5 Oil-Water Separation via FS Laser Structuring
### FS Laser Fabrication of Arrays of Micro-Through Holes  
FS laser microfabrication was used to create rough microstructures on the surface of PTFE sheets, as shown in Figure 10a-c [68]. Following the laser ablation process, the PTFE sheet surface showed a porous structure, comprising highly dense protrusions with heights ranging from 300 nm to 2 \(\upmu\)m. As a consequence, the laser-structured sheets showed both superhydrophobicity and superoleophilicity with static water and oil contact angles of 155.5\({}^{\circ}\) and nearly 0\({}^{\circ}\), respectively (Figure 10d,e). Here, the surface structure enhanced the intrinsic hydrophobic characteristic of PTFE and rendered it superhydrophobic. Subsequently, an array of micro-through holes was formed by mechanical drilling from the backside of the ablated PTFE sheet. As a result, the drilled sheet displayed high efficiency with respect to oil-water separation. Additionally, it was also effective for the separation of strong acid/alkali substances from oil, indicating the high durability and chemical inertness of PTFE sheets in corrosive environments. Figure 9: Oil–water separation with a superhydrophilic–underwater superoleophobic FS laser-treated mesh. Contrary to water, oil is repelled, while both the liquids are collected in separate containers [67]. Reproduced with permission from [67] Royal Society of Chemistry. [PERSON] et al. employed a FS laser with 104 FS pulse duration and a 1 kHz repetition rate to drill an array of micropores on aluminum foil of 2.4-32 \(\upmu\)m diameter and a period of 20-80 \(\upmu\)m [70]. Figure 11a,b presents a typical SEM image of the drilled foil, showing that the micropores' sidewalls are decorated with nanostructures. Although the flat aluminum foil surface is inherently hydrophilic with a water contact angle of 53.9\({}^{\circ}\), the laser-drilled foil became superhydrophobic, showing a water contact angle of 7.8\({}^{\circ}\). As also shown in Figure 11c,d, the oil contact angle of octane and 1,2-dichloroethane was 153.5\({}^{\circ}\) and 153.1\({}^{\circ}\), respectively. For the evaluation of the oil-water separation efficiency, oil-water mixtures were poured into a tube that contained a horizontally placed, laser-processed aluminum foil (Figure 12a-c). During the pouring process, water passed through the micropores, while the oil was inhibited due to the superoleophobic property of the foil. For the separation of heavy oil, a tube with a side opening was used, while the irradiated aluminum foil was placed vertically, as shown in Figure 12d-e. The separation process was completed within 31 s. The continuous separation of oil-water mixtures was additionally demonstrated (Figure 13). In that case, the laser-irradiated aluminum foil was modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (Figure 13a). Owing to such functionalization treatment, the foil wetting properties changed from superhydrophobic-underwater superoleophobic to superhydrophobic-superoleophilic. Figure 13b shows a schematic representation of the continuous separation process. Both of the membranes (i.e., the laser-irradiated and functionalized one) were placed inside a tube with a side opening. It was observed that the water-selective membrane permitted water to pass through, whereas the oil-selective membrane permitted oil to pass through. The whole process was completed within 28 s. As a result, separation can be achieved by using just the gravity effect, with an efficiency of 99% for both light and heavy oil. Figure 10: (**a–c**) SEM of FS laser surface structures on the PTFE surface at different magnifications. (**d**) Image of a water droplet and oil droplet on the PTFE structured surface. (**e**) Water contact angle measurement of the PTFE laser-structured surface [68]. Reproduced with permission from Elsevier 2016. Figure 11: (**a**) SEM image of the micropores drilled in ultrathin aluminum foil by FS laser ablation; the inset shows that light can pass through the micropores’ array.