# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 4 Slippery Liquid-Infused Porous Surfaces (SLIPS)  
A biological model of the considered here biomimetic development is carnivorous plants. Compared to superhydrophobicity, carnivorousness in plants is a relatively rare phenomenon in nature resulting from the adaptation to nutrient-poor habitats. About 600 plant species were identified to having developed interesting morphological and anatomical features related to carnivorousness [83]. One of the most studied pitcher plants is the genus Nepenthes, containing approx. 120 species (Figure 9). These plants evolve special properties of the flower peristome to promote the falling of insects into the flower pitcher. The peristome surrounds the opening of the pitcher and has a unique slippery surface that prevents the escape of prey. The flower peristome is characterized by a regular microstructure [84] whose surface is completely wetted, so that a homogeneous liquid film covers the surface. Only when wet, the peristome surface is slippery for insects, so that most insects coming into contact become trapped [84; 85]. Inspired by this biological model, liquid-repellent synthetic surfaces named \"slippery liquid-infused porous surface(s)\" (SLIPS) were developed by [PERSON]'s group [86]. Instead of using the dry  
Figure 9: Nepenthes pitcher organization (**left**), SEM image of peristome and slippery surface (**middle**), detailed image of slippery zone (**right**). structure surface to repel liquids, the surface structure was used for immobilization of a lubricating liquid that forms a continuous overlying film on the surface. It was presumed that such an intrinsically smooth and defect-free liquid surface can repel immiscible liquids of virtually any surface tension. The design of SLIPS is based on the three following criteria: (1) the lubricating liquid must be stable, immobilized on the substrate surface; (2) the solid must be preferentially wetted by the lubricating liquid rather than by the liquid one wants to repel, and (3) the lubricating liquid and the liquid one wants to repel must be immiscible (Figure 10) [86]. Similar to the development of dry superhydrophobic surfaces, this biomimetic development can also be referred to as biology push or bottom-up. The first SLIPS were fabricated from periodically ordered nanostructured epoxy resin-based and silanized surfaces and a random network of Teflon nanofibrous membranes in combination with low-surface-tension perfluorinated oil. Synthetic SLIPS exhibited extreme liquid repellence, characterized by low contact angle hysteresis <2.5\({}^{\circ}\) and low tilt angle \(\leq\)5\({}^{\circ}\) against liquids with surface tension from 17.2 mNm\({}^{-1}\) (n-pentane) to 72.4 mNm\({}^{-1}\) (water). These slippery surfaces maintained their repellent properties even under high pressure (~676 atm). Moreover, the lubricating film was shown to have a self-healing effect; following damage of the material surface, lubricating liquid refilled the damaged area (voids). It was demonstrated that by matching the refractive indices of substrate and lubricant, SLIPS with enhanced optical transparency in visible and near-infrared wavelengths can be constructed. Nowadays, SLIPS are fabricated on different materials by ultrafast laser ablation. In [87; 88], laser-irradiated porous network microstructures were generated on various polymer surfaces, including poly(ethylene terephthalate) (PET), poly(methyl methacrylate), polyamide, polycarbonate, polyethylene, and polylactic acid. Ultrafast laser-generated structures consisting of protrusions and pores with diameters of several hundred nanometres were subsequently silanized, and infused with silicon oil. Contact angle measurements demonstrated an advancing contact angle of 27.8 \(\pm\) 0.1\({}^{\circ}\) and receding contact angle of 27.2 \(\pm\) 0.1\({}^{\circ}\), resulting in contact angle hysteresis of 0.6\({}^{\circ}\) against a water droplet on PET-SLIPS. A tilting angle of 4\({}^{\circ}\) was measured for water slowly sliding downwards and oil droplets.