Fabrication of bionic reed leaf superhydrophobic surface by laser processing

Abstract

In the natural evolution, animals and plants have evolved many unique structures and properties. For example, butterfly wings appear to be colorful due to grating structures. Lotus leaves have super-hydrophobic property because of micro/ nano-structures. Nepenthes pitcher plants show slippery wettability by reason of the lubricant infused surfaces. Therefore, more and more researchers are trying to prepare unique structures which are similar to the surface of biological templates by different methods. Taking superhydrophobic surfaces as an example, there are many methods having been adopted to achieve superhydrophobic surfaces, such as etching, template method, UV lithography, electrostatic spinning, and nano-imprint technique. In recent years, laser processing technique has the advantages of high precision and arbitrary design, which shows great advantages in bionic manufacturing. For example, various kinds of micro/nano composites or structures have been successfully prepared by laser direct writing, two/four-beam interference, etc. However, these methods rely mainly on complex or sophisticated instruments, which take long time to prepare large-area superhydrophobic surfaces. Reed leaves show anisotropic superhydrophilicity property because of grating structures, which has applications in droplet transmission without loss and directional collection. Herein, we demonstrated the fabrication of bionic reed leaf by laser treatment of structured polydimethylsiloxane (PDMS) surface. Laser treatment of PDMS with micro-grating structures can induce the micro-nano structures and improve the surface roughness. To investigate the change of surface chemical composition, we carried out X-ray photoelectron spectroscopy (XPS) of PDMS before and after the laser treatment. After the laser treatment, the carbon content was decreased from 43.6% to 33.1%. The decrease in carbon content indicates the emission of carbon material during laser treatment, which is caused by the carbonization of the methyl group on the surface of the PDMS. At the same time, the oxygen content increased from 27.8% to 33.6% and the silicon content increased from 28.3% to 33.3%. To compare the surface morphologies changes of PDMS before and after laser treatment, we observed the surface morphology by confocal laser scanning microscopy (CLSM) and scanning electron microscope (SEM). The initial PDMS grating is about 100 μm wide, ~130 μm high and the structure surface is smooth. After laser treatment, it changes to about 200 μm wide, ~110 μm high and becomes much rougher surface. The variation in the width and height is mainly because that the laser treatment causes PDMS surface material to carbonize into rough structures. In order to evaluate the wettability changes of samples, the static water contact angle and dynamic water sliding angle measurement was carried out on the flat PDMS, the structured PDMS, and the laser-treated structured PDMS, respectively. The contact angle along the parallel direction was ∼155°, which is larger than that along the vertical direction (~150°). The sliding angle along the parallel direction was approximately 3°, which was smaller than the SA along the vertical direction (12°). This method provides a new idea for preparation of superhydrophobic surfaces with bionic structures by laser technology. The method is simple, convenient, and low cost, which may become a general method for preparing superhydrophobic surface.