Abstract
Surface geometry has had an influence on the surface property, in addition to the intrinsic surface energy, of materials. Many physical surface modification methods had been proposed to control the solid surface geometry for modification of surface properties. Recently, short-pulse lasers were utilized to perform nano-texturing onto metallic and polymer substrates for the improvement of surface properties. Most of the papers reported that the hydrophilic metallic surface was modified to have a higher contact angle than 120–150°. Little studies explained the relationship between surface geometry and surface properties. In the present study, the laser micro-/nano-texturing was developed to describe this surface-geometric effect on the static contact angles for pure water. Micropatterns with multi spatial frequencies are designed and synthesized into a microtexture. This tailored microtexture was utilized to prepare for computer aided machining (CAM) data to control the femtosecond laser beams. The nano-length ripples by laser induced periodic surface structuring (LIPSS) supposed onto this microtexture to form the micro-/nano-texture on the AISI304 substrate surface. Computational geometry was employed to describe this geometric profile. The fractal dimension became nearly constant by 2.26 and insensitive to increase of static contact angle (θ) for θ > 150°. Under this defined self-similarity, the micro-/nano-textured surface state was controlled to be super-hydrophobic by increasing the ratio of the highest spatial frequency in microtextures to the lowest one. This controllability of surface property on the stainless steels was supported by tailoring the wavelength and pitch of microtextures. Exposure testing was also used to evaluate the engineering durability of this micro-/nano-textured surface. Little change of the measured fractal dimension during the testing proved that this physically modified AISI304 surface had sufficient stability for its long-term usage in air.
Highlights
The surfaces of materials have unique characteristics, different from their body in nature [1]; e.g., a raindrop does not swell on the lotus leaf at the presentation of fine, thin trichomes, and, a water strider skates on the water surface by way of textured feet
Nano-textures by laser induced periodic surface structuring (LIPSS) superposed onto the microtextures, which were directly machined by the femtosecond laser irradiation
The micro-/nano-texturing method is proposed as a femtosecond laser surface modification process for stainless steels
Summary
The surfaces of materials have unique characteristics, different from their body in nature [1]; e.g., a raindrop does not swell on the lotus leaf at the presentation of fine, thin trichomes, and, a water strider skates on the water surface by way of textured feet. The surface property of solid materials is characterized by the surface energy (E) [2]; e.g., when E is high, the liquid drop covers the substrate surface. The drop is isolated on the substrate when E is low. Instead of the direct measurement of E, the swelling angle or the contact angle (θ) is often employed as an indicator of surface energy [3]. The surface state is hydrophilic when cos (θ) > 0 while it becomes hydrophobic when cos (θ) < 0. The surface with cos (θ) ~ 1 is called to be super-hydrophilic, while that with cos (θ) ~ −1, super-hydrophobic [4]
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