Objective Various bioinspired surfaces about super-wettability have been widely investigated. For example, water droplets move freely on the lotus leaf surface, in an anisotropic way on rice leaf surfaces, and unidirectionally on pitcher surfaces. With the progress of science and technology, the mechanisms for these bioinspired surfaces have been revealed. Importantly, bioinspired surfaces have abroad applications in biological, industrial, micromechanical, and other fields. For example, superhydrophobic surfaces, requiring high roughness and low surface energy, show self-cleaning and anti-icing characteristics. From the view of materials, organic polymer materials have lower surface energy than other materials, showing great potential in developing superhydrophobic surfaces. As a typical polymer material, polyvinylidene fluoride (PVDF) shows excellent flexibility, chemical corrosion resistance, and piezoelectricity. Superhydrophobic PVDF has recently been prepared by various methods, such as hybrid modification and surface chemistry modification. However, these methods require special chemical reagents or complicated equipment. Herein, we designed and fabricated PVDF-based membranes with superhydrophobicity by laser processing technology. After the laser treatment, the laser treated-PVDF (L-PVDF) surface owns microstructures and low surface energy. Therefore, the L-PVDF surface shows superhydrophobicity. This work provides a new method to prepare the PVDF membrane with excellent superhydrophobicity.
Methods powders and N, N—dimethylformamide (DMF) solvent are mixed in the ratio of 1 g∶8ml. After ultrasonic treatment for 1h, the PVDF powder is uniformly dispersed. The PVDF@DMF solution is drop-coated on substrates to fabricate PVDF membranes. As for the preparation of L-PVDF surfaces, a continuous semiconductor laser wavelength (λ=450nm, power P=1200mW) is used. After the laser treatment, the L-PVDF surface shows superhydrophobic characteristc. The morphologies of lotus leaf, PVDF, and L-PVDF surfaces are measured by a confocal laser scanning microscope (CLSM) and a scanning electron microscope (SEM). The chemical compositions of the PVDF and L-PVDF are analyzed by X-ray photoelectron spectroscopy (XPS). The surface wettability and wettability stability of the PVDF and L-PVDF are characterized by a static contact angle (CA) measuring system. For the CA measurement, the area of laser treatment is 10mm×10mm, and the processing time is about 3min.
Results and Discussion The static CA of water drops on a lotus leaf is ~150°, indicating superhydrophobic characteristic. To explore the mechanism of the superhydrophobic characteristic of water droplets on the lotus leaf surface, we characterized the lotus leaf surface morphology by the CLSM and SEM, respectively. There are microscaled papillae with a diameter of 3--5μm and a height of 5--10μm (Fig. 1). The existence of microscaled papillae can effectively reduce the contact area between water droplets and lotus leaf surfaces, leading to the superhydrophobic effect. Fig. 2 shows the laser processing system and the procedure of laser processing PVDF surface. The SEM images show that there are grooves along the laser scanning path. The distance between grooves is ~100μm, and the width is ~70μm. Moreover, many particles (diameter is ~ 1μm) are observed. The size and shape of particles are similar to the papillae on a lotus leaf (Fig. 3). Besides, XPS is performed to investigate the change of surface composition of PVDF and L-PVDF surface. The C/F atom ratio has significantly changed from 1.2 (PVDF) to 11.5 (L-PVDF), which indicates that the molecular chain of PVDF is destroyed by high laser power, and defluorination may occur (Fig. 4). Compared with the CA of PVDF film (~ 82°), the L-PVDF surface shows hydrophobicity with a CA of ~ 150°(Fig. 5).
Conclusions Inspired by the microstructures on the lotus leaf surfaces, a L-PVDF-based superhydrophobic surface has been prepared by a continuous semiconductor laser (λ=450nm, P=1200mW). CLSM and SEM are used to characterize the microstructure of L-PVDF. Grooves and microscaled papillae are induced fabrication by the laser thermal effect. The microstructures on the surface of L-PVDF is similar to that of the lotus leaf surface. Besides, the rough structure reduces the contact area between water droplets and the L-PVDF surface. XPS reveals that the C/F atom ratio on the surface increased from 1.2 (PVDF) to 11.5 (L-PVDF). Therefore, the CA of L-PVDF is mainly dependent on the change of microstructures and composition. The static CA on the L-PVDF surface is ~150°. This work shows the fabrication of superhydrophobic L-PVDF films by laser processing. The laser processing is simple and does not involve chemical reagents. We deem that this method provides a new strategy to prepare a PVDF-based superhydrophobic surface.
