An Investigation Into Preparation And Cassie State Stability Analysis Of Multi-scale Superhydrophobic Surface Produced By Laser Ablation And Electrodeposition

Super-hydrophobic surfaces can be found in various animals and plants,including butterfly wings,water striders and lotus.Inspired by biomimetics,the surfaces with special wettability are widely used in self-cleaning,drag reduction,antifogging,and anti-icing.The fabrication methods of super-hydrophobic on metal surfaces have been extensively studied and reported,which are usually required to be modified with a layer of low-surface-energy material after texturing micro/nano structures.However,the investigation into techniques without chemical modification and micro-nano structures’ contribution to Cassie state stability are comparatively limited.In this dissertation,a method for preparing multi-scale superhydrophobic metal surface without modification by laser ablation and electrodeposition is proposed,and the Cassie state stability is also investigated.The micro-cones are formed via picosecond laser ablation efficiently,and then the nanopyramids are electrodeposited on the micro-cones in a short time,resulting in hierarchical structures and 3D re-entrant structures.Based on the design strategy of laser ablation and electrodeposition,the highly ordered micro-cones array protect the nano-pyramids and the latter enrich nano-structures on the former.The main contents were as follows:(1)According to Wenzel and Cassie-Baxter equation,parabolic is the optimized micro-scale structure.Then creative strategy to design micro/sub-micron/nano structure design strategy is proposed.(2)The effect of nano-structures on wettability is investigated by analyzing samples of different processing stages.The micro-structures fabricated by sequential processing(laser ablation and electrodeposition)are similar as those prepared by laser ablation.However,the surface of the micro-cones after electrodeposition are covered by dense sub-micron and nanopyramids,forming multi-scale hierarchical structures and 3D re-entrant structures.The both show high hydrophobicity at room temperature.The contact angles(CAs)of laser ablation and sequential processing are 147° and 160°,and the sliding angles(SAs)are 18° and 1°,respectively.(3)The mechanism of wettability transformation is analysed by comparing surface morphologies,topographies,profiles and compositions.The multi-scale hierarchical structures,hydrophobic NiO film and adsorption of carbon-containing organic compounds provide the superhydrophobicity.(4)The effects of laser parameters on the surface morphologies,size,aspect ratio and wettability of the sample surfaces are investigated.The results reveal that the ordered,regular and high aspect ratio micro-cones surfaces are associated with higher hydrophobicity after depositing nano-pyramids.The optimized laser ablation parameters are the power 30%(9 W),the repetition rate 2 MHz,the scanning interval 20μm,the scanning velocity 200 mm/s,and the scanning number20 times,which the CA is up to 162°and SA at least 1°.(5)The wettability of laser ablated samples and sequentially processed samples with different scanning intervals are measured at normal temperature(25 °C)and low temperature(5 °C).The results indicate that,the hydrophobicity of all samples at low temperatures are reduced.Meanwhile,when laser scanning interval is equivalent to the laser spot diameter,the most stable Cassie state is achieved,as represented in the droplets stamping experiments,where the droplets rebound completely for 3 or 4 times,indicating nano-pyramids and 3D re-entrant structures lead to stable Cassie state.Condensation experiments also demonstrate that the stable Cassie state is closely related to the micro-nano hierarchical structures.Specifically,much smaller water droplets generate on surface with dense nanostructures,and therefore jump away easily from the surface.In addition,when the scanning interval is equivalent to the spot diameter,the microstructures distribute evenly and compactly,resulting in the condensed droplets smaller than the critical size for jumping,and hence a more stable Cassie state.The study carried out in this dissertation not only focuses on efficient fabrication of superhydrophobic surfaces with dense nanostructures and 3D re-entrant structures,but also amply discusses the Cassie state stability of the produced surface,making it promising for practical applications in transportation,communications and military.