# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 1 Introduction
The result of biomimetic process is an invention, which can be a new product or process. The flow chart in Figure 1 represents an ideal case with a linear and sequential procedure [7]. On the way from a new idea to an invention, a stage of prototype construction or manufacturing requires a suitable technology that enables the realization of the biologically based solution in a technical way. At this point, laser-based manufacturing methods can meaningfully contribute to the realization of novel biomimetic prototypes. As will be discussed in this review, particularly in the  
Figure 1: Simplified flow chart of a biomimetic development process, according to [7] (Copyright: Fraunhofer UMSICHT/BIOKON). development of functional liquid-repellent biomimetic surfaces, where specific surface structure at the micro- and nanoscale is one of the key components providing the desired function, we can benefit greatly from ultrafast laser micromachining. Ultrafast means picosecond and femtosecond lasers with a pulse width from several tens of femtoseconds to tens of picoseconds, where a pulse width shorter than picoseconds is usually utilized for fundamental research, while longer pulses are used for industrial applications due to their higher output power and better reliability [9]. Historically, the era of machining by ultrafast laser radiation began to develop intensively after the invention of the post-amplified solid state Ti:Sapphire femtosecond laser. Since that time, the number of publications on ultrafast laser processing has progressively increased, reaching almost half of the papers presented at major specialized conference on laser technologies, including the SPIE LAMOM (Lasers Applications in Microelectronic and Optoelectronic Manufacturing), the Conference on Laser Ablation (COLA), the International Symposium on Laser Precision Microfabrication (LPM), and the European Conference on Lasers and Electro-Optics (CLEO Europe) [9]. Compared to conventional processing using continuous or nanosecond-pulsed lasers, ultrafast laser pulses provide the advantages of precise energy deposition into the processed region and rapid material removal with the suppressed heat diffusion to the surrounding area, resulting in a significantly smaller heat-affected zone and, therefore, better precision and a higher resolution of the generated structures. Ultrafast laser processing offers many benefits in comparison with other structuring techniques used for fabrication surface structures, such as photolithography, electron or ion beam, and mechanical methods:  
* This technique is applicable to almost all solid materials, irrespective of whether they are opaque or transparent, brittle or hard, heat-sensitive or magnetic;
* It enables flexible and controllable structuring of 3D objects with complex geometries;
* Machining is contact-free and can be realized in different environments, such as normal atmospheric conditions, in a vacuum, in different gases or liquids;
* There is no need for special expensive facilities;
* It enables the fabrication of a large variety of surface structures. Physical processes underlying the interaction between ultrafast laser radiation and different types of materials have been described in several key publications in recent years [10; 11; 12; 13; 14; 15]. Surface structures generated with ultrafast laser radiation can be roughly divided into two main groups: laser-inscribed and laser-irradiated structures (Figure 2) [16]. The structures that were intensively used for the fabrication of biomimetic liquid-repellent surfaces are marked in gray. Figure 2: Classification of laser-patterned surface structures; structures that were intensively used for the fabrication of biomimetic liquid-repellent surfaces are marked in gray, modified according to [16]. As will be seen below, through the fabrication of laser-inscribed structures a nearly any structure can be realized (see Section 2.1).