Study On Roll-to-Roll Embossing Of Shark-skin-inspired Drag-reduction Micro-riblets

Along with the increasing problem of energy, it has become a hot topic in the drag-reduction field that imitating the micro-riblets of shark skin to reduce the drag. However, due to the disadvantage of complex-structure, hard-replicate, low-precision and high-cost of shark skin, it is difficult and challenging to replicate the shark-skin-inspired micro-riblets. In order to solve this problem, something was studied in this paper.Firstly, the structures and sizes of shark-skin-inspired micro-riblets were attained with the simplification and optimization of actual micro-structures of shark skin. Then, the drag reduction property of the micro-riblets was analyzed. The results indicated that the surface of shark-skin-inspired micro-riblets showed the obvious drag reduction effect. The main reasons were that the fluid near the riblets flows gently and there is vortex in the bottom of the riblets.Secondly, based on the hydrophobic theory, a new hydrophobic model which is consistent with the special structure of shark-skin-inspired micro-riblets was proposed. Then, the contact angles of different droplets were measured by optical contact angle measuring device on shark-skin-inspired micro-riblets and the smooth surface, respectively. The results showed that the surface of micro-riblets possessed the obvious hydrophobicity, and the actual contact angles of different droplets residing on the riblets decreased with the increase of the droplet volume. According to the new hydrophobic model and the measurement of contact angle, it was found that the hydrophobic property of shark-skin-inspired micro-riblets was the result under the synthetic action of arrangement and structure of the micro-riblets. By using the new hydrophobic model, the prediction error of contact angle can be less than3%compared with the measured one.Thirdly, taking the size of shark-skin-inspired mold, the cost and the difficult degree of manufacturing into consideration, the metal molds with shark-skin-inspired micro-riblets were attained by UV-LIGA technology and high-speed milling. The structures’sizes of the mold made by UV-LIGA technology were similar to the micro-riblets’sizes of real shark skin. The structure outlines were clear. And the electroforming deposits were compact. The mold structures made by high-speed milling were clear-cut, and surface roughness was small. The processing results of molds were clear-cut and the structure sizes were completely satisfied for present experiment. The feasibility of manufacturing the mold with these two methods was validated.Finally, after designating the experimental principle, experimental equipment, experimental materials and process parameters, the Roll-to-Roll embossing experiment of shark-skin-inspired micro-riblets on the surface of PVC and PET were completed. And the influence of mold temperature, the roller speed and pressure on the forming results was studied. The research showed that Roll-to-Roll embossing process was a dynamic and progressive forming process. Because of the long-time cooling of polymer film, the Roll-to-Roll embossing process should be accomplished under the conditions of low-temperature, high-pressure and low-speed. The mold attached to the roller could not only accomplish continuous embossing, but also complete better embossing results.