# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 4 Slippery Liquid-Infused Porous Surfaces (SLIPS)
Therefore, among all tested titanium surface structures only the columnar-structured titanium possesses the potential for stable SLIPS. Finally, durability of columnar-titanium-SLIPS coated with different lubricants under rotational forces (aqueous shear stress for 5 h at 150 rpm) was tested. Optical coherence tomography showed that lubricant layers could only be detected for the lubricants of intermediate viscosity (143 AZ, GPL 104, GPL 105). It has already been shown that physical forces, such as dynamic flow or centrifugal forces, may remove lubricants from SLIPS [92; 93; 94]. A study [90] experimentally demonstrated that there exists a definite range of surface structure dimensions and lubricant viscosities leading to stable SLIPS. Lubricants with very low viscosity are not inert enough to withstand physical and energetic forces, whereas lubricants with very high viscosity are not fully trapped by the surface structures. Technical applications of SLIPS fabricated by ultrafast laser processing include self-cleaning and antifouling. For example, in [87; 88] polymer-SLIPS were shown to repel a broad range of liquids, such as water, ink, glycerol, coffee, milk, fruit juice, egg white, and egg yolk from tilted surfaces under normal ambient conditions. In 89] the anti-fouling properties of steel SLIPS were tested in a real industrial pasteurization scenario in view of their ability for dairy fouling mitigation. In the dairy sector thermal treatments are mandatory to ensure product safety. The result of the thermal treatment is heat-induced fouling, forming an essential part (up to 80%) of the total production costs. Steel-SLIPS in industrial-like conditions demonstrated significant antifouling performance, with a contamination weight reduction of 63 \(\pm\) 4 wt % compared to native stainless steel. Moreover, tests showed that a 20-min rinsing step with water was sufficient to eliminate all traces of fouling from the steel-SLIPS surface. In comparison, a multiple-step procedure consisting of a pre-rinse, followed by an alkali cleaning, an intermediate rinsing step, an acid cleaning, and a final rinsing to achieve the same result on native stainless steel. For durability tests, steel-SLIPS were tested and rinsed repeatedly with and without oil reimpregnation in between. Re-impregnated steel-SLIPS showed the same performance as at their first use. Samples without reimpregnation exhibited reduced contamination weight reduction of only 26 \(\pm\) 12 wt %, which was attributed to oil shedding from the surface. To sum up, this study clearly demonstrated the potential of steel-SLIPS against heat-induced dairy fouling; however, the problems related to dairy products' contamination with oil and the improvement of oil retention should be addressed in further investigations. Another important application tested on titanium-SLIPS fabricated using ultrafast laser processing is the prevention of biofouling in medicine--for example, inhibition of bacterial colonization. In [25], columnar-titanium-SLIPS coated with lubricants of intermediate viscosity (143 AZ, GPL 104, GPL 105) showed statistically significant reduction in adhering bacteria (_S. oralis_) \(\sim\)10-fold compared to that of the unstructured, uncoated control (Figure 13). For live/dead bacteria distribution, no statistical differences could be detected compared to the control. This supports the hypothesis that SLIPS prevent initial cell attachment rather than inhibit biofilm growth. Tests with human fibroblasts and osteoblasts demonstrated that both cell types were not able to adhere to slippery surfaces. Also in this case, live/dead fluorescence staining has indicated that cells were viable [90]. In [88], C6 glioma cells were tested on flat, structured, and slippery PET surfaces (silanized and infused with silicon oil). The number of adhered cells was significantly increased on structured (\(\approx\)3469 mm\({}^{-2}\)) compared to flat (\(\approx\)1349 mm\({}^{-2}\)) PET surfaces. In contrast, nearly no detectable adhered cells were observed on PET-SLIPS [88].