| Control Function of Butterfly Wings Specialty:Bionic Science and Engineering Advisor:Professor Zhiwu HanWith the ceaseless growth of the population all over the world,the continuous improvement of the industrialization and the gradual aggravation of environmental pollution,the problem of water shortage is becoming increasingly severe.How to efficiently obtain fresh water resources has become a hot topic.Liquid control technology is one of the important strategies to alleviate the crisis of water resources shortage.Typical liquid control technologies include steam generation technology,fog collection technology,liquid transportation technology and oil-water separation technology.On the one hand,with the help of steam generation technology and fog collection technology,water can be extracted from the water system in nature by controlling liquid phase transformation to obtain renewable fresh water resources.So,these two technologies could broaden water sources of income.On the other hand,with the help of liquid directional transportation technology and oil-water separation technology,the position and composition of liquid can be changed and controlled.This can complete the proper allocation of existing water resources and the recycling of oily wastewater,which could avoid the waste of water resources and further deterioration of water pollution.Therefore,these two technologies could reduce expenditure.Through the application and promotion of the above four key liquid control technologies,a virtuous circle of the development and utilization of water resources can be formed.The promotion and application of typical liquid control technology relies on the design and preparation of liquid control functional materials.After several centuries of efforts,researchers have developed a large number of liquid control functional materials.However,the following technical problems exist in traditional liquid control functional materials.First,Low photo-thermal conversion efficiency and poor mechanical stability of steam generation materials(mainly due to weak binding force between photo-thermal conversion nanoparticles and substrate materials).Second,the low efficiency of fog collection and liquid directional transportation materials results in the low speed of water collection and liquid transportation,which is far from meeting the demand for water of human daily production and survival.Third,the mesh structure of oil-water separation material is easy to be blocked by oil with high viscosity,thus affecting the efficiency of oil-water separation.In conclusion,liquid control functional materials still have a significant potential for growth in the design and preparation.As we all know,the excellent properties of materials not only depend on the chemical composition,but also on the structure.Butterfly wings have gradually formed a surface structure with excellent liquid control performance in billions of years of natural selection and evolution.The excellent liquid control performance of butterfly wing surface structure is much better than the existing liquid control functional materials.These natural structures provide new ideas and new references for the design and development of new biomimetic liquid control functional materials.In this paper,the butterfly species living in the typical areas are selected as the biological model.Combined with the principle of bionics,the liquid control mechanism of butterfly wings is revealed by experimental characterization and theoretical analysis.Besides,the visualization models of multi-scale hierarchical structure of butterfly wings is constructed.Based on these models,four kinds of biomimetic liquid control functional materials are designed and prepared.In addition,the performance test is also carried out.The main contents of this paper are as follows:1.Characterization of multi-scale hierarchical structures and liquid control capabilities of butterfly wing,including highly integrated and efficient photo-thermal conversion of honeycomb structure coupled with carbon based biomass of butterfly wing,excellent wettability amplification function of ridge array-fold texture hierarchical structures,directional rolling transport of droplets on ratchet structure with alternating characteristics of hydrophilic and hydrophobic,anti-oil property of parallel rib structure coupled with super-wetting functional groups.(1)The characteristic structure and key size parameters of the scales or bristle array on the wing surface were characterized by stereomicroscope,superfield 3D imaging system,scanning electron microscope and laser confocal microscope.Macroscopically,there are alternate overlapping scales or bristle array on the surface of the butterfly wings.Microscopically,the scales or bristles possess a multi-scale hierarchical structure,including: microscale primary structure,such as periodic ridge array and randomly distributed holes,and nanoscale secondary structure,such as stacked lamellae and folded textures.(2)With the help of optical testing platform,contact angle measuring instrument,simulated spray device and high-speed camera system,the liquid control behavior and key liquid control parameters of multi-scale hierarchical structure were observed.First,the combination of honeycomb structure and carbon based biomass shows excellent light trapping characteristics,which improves the photothermal conversion efficiency of butterfly wings.Afterward,the coupling of microscale ridge array and nanoscale fold textures with hydrophobic biomass endows butterfly wings with superhydrophobic characteristics,which confirms the wettability amplification function of multi-scale hierarchical structure.Besides,with the increase of droplet size on the bristle surface,the hydrophilic/hydrophobic alternating ratchet structure completes the droplet transport along the bristle from bottom to top overcoming the resistance of gravity.Finally,the parallel rib structure coupled with its special wetting functional group makes the butterfly wing possess anti-oil property.2.Reasons for formation,influencing factors and mechanism of excellent liquid control performance of multi-scale hierarchical structure on butterfly wings.First,the honeycomb structure forms a large number of inner cavities distributed in the wing membrane.The multiple reflection mechanism of the inner cavity structure to the sunlight increases the light path and the number of light energy conversion.Therefore,the integration of honeycomb structure and carbon based biomass shows excellent photothermal conversion characteristics.Secondary,the microscale ridge array and nanoscale fold textures significantly improve the specific surface area and surface roughness of the butterfly wing,coupling with the hydrophobic component,the butterfly wing possesses a special wettability with the help of wettability amplification of the multi-scale hierarchical structure.Third,the hydrophilic/hydrophobic alternation material property endows the bristle surface with hydrophobic adhesion characteristic.Ratchet structure cooperates with its hydrophobic adhesion characteristic makes the bristle exert unidirectional riveting force on the droplet.When the droplet size increases or the droplets merge,the droplet can only move to the upper end of the bristle,and finally realize the directional transport of the droplet.Forth,the parallel rib structure forms a large number of liquid channels on both sides of the ridge structure unit.The special wetting functional groups on its surface can selectively make the oil pass through and repel the water out of the channel.At the same time,the existence of the parallel rib structure enhances the surface area density and oil volatilization rate.Therefore,the butterfly wing has the degreasing characteristic and oil-water separation function.3.The biomimetic design of the structure model for the liquid control functional materials,including honeycomb structure model,ridge array-conical cylindrical structure unit hierarchical structure model,ratchet structure model and dendritic structure model.According to the interfering factor and rules of the liquid control characteristics of butterfly wings,four kinds of multi-scale hierarchical structure and liquid control function models are optimized and designed based on the principle of coupled bionics,including light trapping property and integrated high efficiency photothermal conversion capability of honeycomb structure model,ridge array and conical columnar unit hierarchical structure model coupled with hydrophilic material shows super wetting characteristics and high efficiency fog capturing capability with the help of wetting amplification mechanism,a ratchet structure model with hydrophobic adhesion realizes directional rolling transport of droplet under unidirectional riveting force,The high surface density property of tree like structure model coupled with the special wetting functional groups shows degreasing property.4.The preparation and performance test of liquid control functional materials with the biomimetic multi-scale hierarchical structure.Based on the above multi-scale hierarchical structure models and liquid control mechanism of butterfly wings,four kinds of liquid control functional materials were prepared,and the morphology and structure of the materials were characterized.The results show that the liquid control material has a multi-scale hierarchical structure similar to the butterfly wing prototype,achieving the goal of "similarity in form" bionics.In addition,according to different applications,performance test devices for biomimetic liquid control material are designed and built.The test results show that,in terms of performance,the liquid control functional material is consistent with the butterfly wing,so as to achieve the ultimate goal of " similarity in inherence" bionics. |
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