# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 4 Slippery Liquid-Infused Porous Surfaces (SLIPS)
Additionally, PET-SLIPS were shown to have high resistance to bending and friction. In [89], the fabrication of cauliflower-like slippery liquid-infused surfaces on steel using ultrafast laser processing was reported (Figure 11). After subsequent UV/ozone treatment, which was applied to remove organic contamination and generate hydroxyl groups, surfaces were silanized. Steel-SLIPS were prepared by surface infusion with Krytox 103 (DuPont) perfluorinated oil. By wettability characterization of steel-SLIPS the following data were obtained: water contact angle: 116.6\({}^{\circ}\)\(\pm\) 1.3\({}^{\circ}\), contact angle hysteresis: 0.6\({}^{\circ}\)\(\pm\) 0.2\({}^{\circ}\), and tilting angle: 2\({}^{\circ}\). Figure 10: Fabrication of slippery liquid-infused porous surfaces (SLIPS) by infiltrating a functionalized textured solid with a lubricating liquid to form a physically smooth and chemically homogeneous lubricating film on the surface, enabling the test liquid to easily slide down on the tilted SLIPS. In [90], different laser-fabricated structures (hierarchical columnar and ripple structures, as well as laser-inscribed groove structures, Figure 12) and five lubricants with different viscosities (Krytox GPL 100 (4 cSt at 40 \({}^{\circ}\)C), Krytox 143 AZ (18 cSt at 38 \({}^{\circ}\)C), Krytox GPL 104 (60 cSt at 40 \({}^{\circ}\)C), Krytox GPL 105 (160 cSt at 40 \({}^{\circ}\)C), and Krytox GPL 106 (240 cSt at 40 \({}^{\circ}\)C)) were tested in view of their SLIPS performance on titanium surfaces. Figure 11: SEM images of native (**A,B**) laser-irradiated structure on steel (**C,D**), which was used for SLIPS fabrication. (Reprinted with permission from [89]. Copyright 2017 American Chemical Society.) Before the spin-coating with lubricants, titanium surfaces were coated with a fluorinated polymer On all titanium-SLIPS, regardless of the structure and lubricant applied, a static water contact angle of 100\({}^{\circ}\) was measured. The water contact angle hysteresis below 5\({}^{\circ}\) on columnar-titanium-SLIPS for all tested lubricants was observed. For groove-, ripple-, and unstructured-titanium-SLIPS, contact angle hysteresis was also lower than 5\({}^{\circ}\) for more viscous lubricants (GPL 104, GPL 105, and GPL 106), and could not be measured for GPL 100 and 143 AZ due to pinning of the water droplet by tilting the surface. For a closer investigation, different titanium structures were infused with 143 AZ or GPL 104 and analyzed by optical coherence tomography. In all cases, the lubricant's thickness is approximately 60-85 \(\upmu\)m but varies according to the underlying structure and lubricant. It was found that the thickness of 143 AZ lubricant layers was \(\sim\)20 \(\upmu\)m lower than the corresponding thickness of GPL 104 layer on all samples. It is noteworthy that the lubricant's thickness on columnar-titanium-SLIPS was approximately 10 \(\upmu\)m greater than that on the corresponding groove-, ripple-, or unstructured-titanium-SLIPS samples. The durability of different titanium-SLIPS structures infused with 143 AZ or GPL 104 was tested using optical coherence tomography after incubation in an aqueous environment (biofilm cultivation medium) for 18 h. It was found that the thickness of the lubricant layer on columnar-titanium-SLIPS was reduced by 20-30% due to immersion, leaving a film of 60 \(\upmu\)m thickness on the surface. On SLIPS fabricated on grooves, ripples, and unstructured titanium surface, complete removal of the lubricant film was observed. This is the result of lubricant separation and building of spheres in hydrophilic environment, such as the biofilm cultivation medium or PBS, to decrease the overall free energy [91]. It was assumed that only the columnar structure thermodynamically favors lubricant immobilization on the surface against hydrophobic sphere formation in a hydrophilic environment, probably due to the specific hierarchical nature of the structure.