# Biomimetic Liquid-Repellent Surfaces by Ultrafast Laser Processing
## 1 Introduction  
This article gives an overview of the biomimetic liquid-repellent surfaces produced by ultrafast laser radiation. During the last few decades, both of these fields: biomimetics and ultrafast laser technologies have been developed very intensively. The exponential growth of biomimetics is reflected in a steadily rising number of scientific publications in this field, from tens of papers in the mid-1990s to nearly 2500 papers per year as of 2017 [Web of Science]. As an introduction to this topic, the following books can be named [1, 2]. The subject area of biomimetics doubles every 2-3 years, exceeding the modest expansion of about 6% per year for science in general [3]. Progress in technological transfer of biomimetic structures can be clearly seen in the rapid growth in the number of patents granted in this field [4, 5]. However, the term \"biomimetics\" is not always used unambiguously and correctly in numerous scientific publications. Therefore, firstly a brief remark on terminology in biomimetics is given. Originally, the word \"biomimetics\" was introduced by an American inventor, engineer, and biophysicist [PERSON] in the 1950s. He made a distinction between physics and engineering approaches to the biological sciences, which was defined as \"biophysics\", and a biological approach to engineering, which was defined as \"biomimetics.\" Recently, the ISO/TC 266 Biomimetics committee has given the following definition for the term Biomimetics: \"Interdisciplinary cooperation of biology and technology or other fields of innovation with the goal of solving practical problems through the function analysis of biological systems, their abstraction into models and the transfer into and application of these models to the solution\" [6]. Based on the guidelines of the German Association of Engineers (Verband Deutscher Ingenieure, VDI) [7], a technical development can be referred to as biomimetic if it fulfills the following criteria:There is a biological model (which is linked to a technical question). 2. The properties of the biological model are analyzed and abstracted. (Analysis means a systematic examination of the system by its decomposition into elementary components using a suitable method following their subsequent evaluation and reorganization. Abstraction can be understood as an inductive process in which a general conclusion is made in the form of physical relationships based on the observation of the biological model.) 3. Abstracted biological results are transferred to a technical application. In this context, both biological species with particular properties (materials, structures, functions, processes) and the process of evolution itself could be considered as biological models. There are many reasons to use ideas \"made by nature\" for technical applications. Nature has already invested 3.5 billion years of evolution in the development of highly optimized biological models. The result of this development is some 10 million living prototypes (i.e., the estimated number of existing plants and animals) [8]. Last but not least, the same physical laws and constants are valid in biology and in technology. The essence of biomimetics is interdisciplinary and transdisciplinarity. It requires a high level of cooperation between experts from different fields of research such as biology, chemistry, physics, and engineering, leading to innovative, cross-disciplinary methods and tools. The typical methodology used in biomimetics is presented in Figure 1[7]. The starting point for new development in biomimetics can be either initiated from basic biological research or from a specific technical problem. The first approach is referred to as the biology push bottom-up process. The second one is referred to as the technology pull top-down process. As a next stage, the biological model is analyzed using methods from the natural sciences as well as engineering sciences and abstracted. The phases between planning and invention involve typical steps of product or process development.