# Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil-Water Separation
## 5 Oil-Water Separation via FS Laser Structuring  
Oil-water separation was realized for the first time via a superhydrophobic and superoleophilic mesh. In particular, a rough microstructured polytetrafluoroethylene (PTFE) coating deposited on stainless mesh was used for separation [54]. It was reported that when an oil-water mixture is poured on the mesh, oil gets absorbed, while water is repelled due to the superhydrophobic-superoleophilic behavior of the prepared mesh. Over the past decade, superhydrophobic-superoleophilic and superhydrophilic-underwater superoleophobic membranes have been produced using different techniques on various materials. Such techniques include electrochemical deposition, sol-gel process, chemical etching, lithography, spray and dip coating; and they have been employed to modify materials such as filter paper [55], cotton [56; 57], fabric [58], wood [59], metallic meshes [60], and sponges/foams [55; 61] for oil-water separation [62; 63]. Moreover, oil-water separation has been reported for superhydrophilic-underwater superoleophobic surfaces [64]. Although such techniques have provided promising separation efficiencies, it was impossible to retain their special wetting characteristics for a prolonged time. This can be attributed to the corrosive action of the liquids, high temperature, friction, and mechanical wear and tear [65]. Moreover, most of the produced coatings were fragile and easily worn off by slight mechanical abrasion; as a result, the super-wetting properties significantly degraded over time. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **Laser Structures** & **Fabrication Parameters** & **Chemical Treatment** & **Wettability Characteristic** & **Oil Separated** & **Ref.** \\ \hline Janus mesh & LISR & 580 mW, 7.39\(\times\) & \multicolumn{4}{c}{**FS Laser-Structured Meshes**} \\ (side 1) & LISR & 10\({}^{7}\) m/s/cm\({}^{2}\) & 11H,1H2H2H2-perfluorodecytriethoxysilane & Superhydrophobic-superoleophilic & Ben oil, n-heptane, methyl benzene & [66] \\ Janus mesh & - & - & Graphene oxide & superhydrophobic-underwater & Perchloromethane, trichloromethane. \\ Stainless steel & LISR & \(\lambda=1030\) nm, 250 & superhydrophobic-underwater & Edible oil, diesel, crude oil, & [67] \\  & & FS, 75 kHz & - & superhydrophobic & heasdeane, 1,2-dichlorethane. \\ \hline \multicolumn{6}{c}{**FS Laser-Fabricated Arrays of Micro-Through Holes**} \\ \hline PTFE & - & \(\lambda=800\) nm, 50 FS, & \multicolumn{4}{c}{} \\  & \(\lambda=800\) nm, 50 FS, & \multicolumn{4}{c}{} \\ Titanium & - & 1 kHz, 31–15.5 & - & superhydrophobic & petroleum ether, heptane, hexane, & [68] \\  & & \(\lambda=800\) nm, 104 FS, & \multicolumn{4}{c}{} \\ Aluminum & - & 1 kHz, 4–5 & \multicolumn{4}{c}{} \\  & & pulses/hole & \multicolumn{4}{c}{} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Oil–water separation filters/meshes realized by FS laser fabrication. On the contrary, FS laser microfabrication has shown exceptional results with respect to the realization of durable and reliable superwetting materials for efficient oil-water separation [62]. Table 1 summarizes the most important recent reports on this topic. In particular, oil-water separation filters have been formed by either FS laser drilling of micro-through holes in a thin sheet or growing hierarchical microstructures on a mesh surface via FS laser ablation. Oil-water separation has been realized with or without subsequent chemical functionalization [11; 69; 71]. Recently, FS laser surface structuring has been used to produce six different types of wettability for polydimethylsiloxane (PDMS) and aluminum surfaces [72; 73]. FS laser-irradiated aluminum surfaces exhibit superhydrophilic, underwater superoloepphobic, and underwater superaerophilic wettability characteristics. When the FS laser-structured aluminum surface is functionalized with 1H,1H,2H,2H-perfluorodecyltriethoxysilane, its wettability behavior changes completely to superhydrophobic, underwater superoleophilic, and underwater superaerophilic [72].