Abstract
Aircraft icing has a significant impact on flight safety， as ice accumulation on airfoils and engines can cause aircraft stalls. Developing anti-icing technology that can adapt to harsh and cold environment presents a challenge. Here， we propose a new anti-icing skin with micro-nano structure inspired by the bamboo leaf called Fargesia qinlingensis. A multilayer non-uniform height （MNH） micro-nano structure is proposed based on the Fargesia qinlingensis surface structure. The anti-icing mechanism of the MNH micro-nano structure is revealed. The flexible large-area MNH micro-nano structure is fabricated based on hierarchical assembly method. Compared with the smooth surface， the ice adhesion strength of the prepared bio-inspired surface is reduced by 80%， indicating that the MNH micro-nano structure inspired by Fargesia qinlingensis has ice-phobic effect. Based on this， an anti-icing hybrid skin based on bionics and electric heating is developed. The anti-icing hybrid skin has successfully completed the anti-icing function flight test on the UAV. To realize the effective anti-icing function under super cold conditions， the anti-icing hybrid skin has been applied on a certain type of UAVs. The bio-inspired anti-icing skin has broad application prospects in large transport aircraft， helicopters， wind power generation， and high-speed trains.
Aircraft icing has great impact on the flight safety， and the anti-icing technology for aircraft in harsh and cold environment is a challeng
The biomimetic micro-nano structure surface provides a promising direction for anti-icing researc
Many unique phenomena in nature are the results of thousands of years of evolution to adapt to the environment. Although the bionic superhydrophobic surface has the anti-icing effect， these plants are not in direct contact with ice and snow after all. We find that there is a unique plant in Qinling Mountains─Fargesia qinlingensis， which has a surface that is not easily adhered to ice and snow， as shown in
Fig.1 Anti‑icing performance of Fargesia qinlingensis leaves
A multilayer non-uniform height （MNH） micro-nano structure is proposed based on the Fargesia qinlingensis surface structure. The anti-icing mechanism of the MNH micro-nano structure is elucidated， and a fabrication method based on hierarchical assembly is provided for preparing flexible， large-area MNH micro-nano structures. An anti-icing skin based on bionics and electric heating is developed. The anti-icing skin has successfully completed the anti-icing function flight test on small-to-medium-sized UAVs， and it has been applied on a certain type of UAVs.
The surface structure and chemicals of the Fargesia qinlingensis leaves are analyzed. The leaves are from the watershed at Fengyu valley in Qinling Mountains. Plants in good growth condition are selected， and 5 leaves with the length of no less than 4 cm are selected from each plant. Leaf samples are washed with ethanol immediately after harvest， and sealed in test tubes filled with deionized water. The time from harvest to experiment is less than 6 h to ensure freshness. Scanning electron microscopy （VEGA3， SBH， TESCAN Company Ltd） is used to observe the morphology of leaf samples， and the results are shown in
Fig.2 SEM images of MNH micro-nano structure of Fargesia qinlingensis
The unique MNH micro-nano structure of Fargesia qinlingensis is the key to its ability to repel ice. For common regular array microstructures， supercooled microscopic water droplets will infiltrate into the gaps of the structure， which will weaken the superhydrophobicity and lead to surface icing. At the same time， the ice embedded in the structure will also produce the mechanical interlocking， increasing the ice adhesion strength. However， the MNH micro-nano structure of Fargesia qinlingensis is conducive to the bouncing of supercooled water droplets， and enhances the anti-wetting ability. Even if some water droplets fail to leave the surface in time and freeze eventually because of the high liquid water content and strong airflow， the ice adhesion strength is relatively low. This is because the multilayer and multiscale micro-nano structure forms more cavities， and the ice on top is more likely to generate local stress concentration. Therefore， the ice-solid interface tends to produce micro-cracks， which effectively reduces the adhesion strength. The schematic diagram of anti-icing mechanism of Fargesia qinlingensis is shown in
Fig.3 Schematic diagram of the anti-icing mechanism of Fargesia qinlingensis
The fabrication of large-scale and complex topographical structures has always been a challenge for micro-nano processing technology. The topography of Fargesia qinlingensis involves structures at different scales. Using a single mask etching method cannot produce multilayer micro-nano structures， and adding masks to achieve structures with different heights is also difficult. Here， we propose a fabrication method based on hierarchical assembly to realize a flexible large-area MNH micro-nano structure. First， large-area single-layer structure surfaces are prepared. Then， the layers are combined by adjusting the interfacial adhesion force， and a large-area flexible multi-layer structure is obtained. The schematic diagram of the fabrication process for the MNH micro-nano structure is shown in
Fig.4 Fabrication process of MNH micro-nano structures
Patterned photoresist structures on the nickel substrate are obtained through photolithography， and then the nickel mold master with fine microstructure is obtained through electroforming. Multiple nickel molds of the same size are obtained by secondary electroforming. An aligner is used to realize the preparation of large-area negative templates （Step A）. Furthermore， large-area single-layer microstructures are obtained on the polyimide substrate by imprinting and photocuring （Step B）. Here， the drop-like structure （Structure A） is fabricated differently from the papillae structure （Structure B）. More specifically， for the drop-like structure， a pre-prepared smooth and thin ultraviolet-cured polymer （UV-cured polymer） is placed in 1H， 1H， 2H， 2H-perfluorooctyltriethoxysilane solution for 2 h， and is dried in the oven with 60 ℃， 30 min. The adhesion of the silaneized surface is reduced due to the formation of C
The SEM image of the simplified structure inspired by Fargesia qinlingensis is shown in
Fig.5 SEM image of the prepared MNH micro-nano structure
The ice adhesion strength of the prepared MNH micro-nano structure is measured by a self-made adhesion force measuring equipment with the ambient temperature -10 ℃， and the water droplet is 15 μL. After the water drop freezing on the surface， the force probe is driven by the motor to remove the ice， and the force curve is recorded. According to the image method， the contact area between the ice drop and the surface is obtained， and the ice adhesion strength is calculated.
Fig.6 Results of ice adhesion strength of MNH micro-nano structure and smooth surface
Previous research has proved that under the impact of high-speed cold airflow， it is difficult to achieve long-term and efficient anti-icing effect only by passive anti-icing technology. The consensus is that a combination of active and passive anti-icing technologies is currently the most reliable solution to aircraft icing problems. Therefore， we propose an anti-icing scheme， adding a surface insulation layer， a metal heating layer and a bottom insulation layer under the bionic anti-icing structure layer to obtain an anti-icing hybrid skin. The bionic anti-icing structure layer can prevent the accumulation of supercooled water droplet， and the electric heating layer ensures the efficient anti-icing process in extremely harsh environments. The upper and lower insulation layers are both PI films. The metal heating layer is made from constantan and fabricated by photolithography and etching. Constantan has high resistivity and thermal conductivity （4.8×1
When an aircraft passes through a supercooled cloud， the impact of supercooled water droplets in different areas of the airfoil is various. Adopting a single electrothermal power density cannot achieve a reasonable distribution of energy and may even cause shortage for aircraft energy supply. Therefore， it is necessary to perform chordwise power density partitioning on the electric heating of the skin to reduce anti-icing energy consumption and ensure a longer endurance time. A low energy consumption bionic anti-icing system based on power density partition is proposed. The numerical calculation method is used to analyze the anti-icing power density distribution regularity of the airfoil surface under specific working conditions， and the anti-icing hybrid skin is designed according to the calculated power density distribution.
Considering the actual flight requirements， the energy consumption requirements for the UAV anti-icing system are the most stringent among other aircraft. Here， a certain type of the UAV is selected as the experimental object for research. According to the flight altitude， speed， attack angle of the UAV， and the meteorological design standards under typical conditions in Appendix C of CCAR-2
Fig.7 Design of anti-icing hybrid skin
Considering the anti-icing effect of the MNH micro-nano structure，the accumulation of water droplets is decreased， leading to the reduction of the heat flow related to water droplets. Therefore， the correction coefficient is used here to characterize the influence of the MNH micro-nano structure surface on the calculation of the anti-icing heat flow. Experiments on the capture rate of supercooled water droplets are carried out on the surface of MNH micro-nano structure in a low temperature environment. The experimental results show that under the working conditions mentioned above， the MNH micro-nano structure surface can reduce the water collection by about 90% compared with the ordinary surface. The corrected theoretical anti-icing thermal load is shown in
The anti-icing performance of the anti-icing hybrid skin is verified in the ice wind tunnel. The anti-icing hybrid skin is attached to the surface of the airfoil， and the test conditions are shown in
| Wind speed/(m · | Temperature/℃ | MVD/ (g · | LWC/ (g · | Pressure/MPa | Water pressure/MPa |
|---|---|---|---|---|---|
| 40 | -10.1 | 18.3 | 0.503 | 0.15 | 0.1 |
Fig.8 Ice wind tunnel test results for anti-icing hybrid skin
The flight test of the developed anti-icing hybrid skin demonstrates that the tested UAV successfully completes the flight mission at an altitude of 1─4 km， a maximum flight altitude of 8 km， a cruising speed of 40 m/s and a temperature of -10─-3 ℃. At present， the anti-icing hybrid skin has been successfully applied on a certain type of UAVs， becoming the world’s first mid-airway long-endurance UAV with anti-icing function.
Fig.9 Photo of the flight test of the anti-icing hybrid skin
Aircraft icing poses a great threat to flight safety， and the bionic micro-nano structured surface provides a new direction for anti-icing technology. A unique plant in Qinling Mountains with excellent anti-icing effect called Fargesia qinlingensis is found. The MNH micro-nano structure inspired by Fargesia qinlingensis is proposed， and the anti-icing mechanism of the multilayer structure is discussed. The multilayer and multiscale structure enhances the bouncing behavior of supercooled water droplets， and the large number of cavities reduce ice adhesion by facilitating formation of micro-cracks on the interface. A method for the preparation of flexible large-area micro-nano structures based on hierarchical assembly is proposed. The monolayer is fabricated through imprinting and photocuring， and surface modification by fluorosilanes and corona discharge is applied to realize the combination of the layers. The ice adhesion strength of the obtained anti-icing MNH micro-nano structure surface is reduced by 80%， comparing with the smooth surface. Considering the actual flight environment， a low energy consumption bionic anti-icing system based on power density partition is proposed. The UAV flight test demonstrates that the developed anti-icing hybrid skin has great anti-icing performance at an altitude of 1─4 km， a maximum flight altitude of 8 km， a cruising speed of 40 m/s， and a temperature of -10─-3 ℃. The developed anti-icing hybrid skin has been applied on a certain type of UAVs. The bio-inspired anti-icing skin has broad application prospects in large transport aircraft， helicopters， wind power generation， and high-speed trains.
Contributions Statement
Ms. YAN Zexiang contributed to the design and preparation of the material and wrote the manuscript. Prof. HE Yang conducted the analysis and experiments. Prof. YUAN Weizheng contributed to the discussion and application of this study. All authors commented on the manuscript draft and approved the submission.
Acknowledgements
This work was supported in part by the National Natural Science Foundation of China （Nos.51875478， 51735011， 52111530127） and the Foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research （No.61422010102）. The authors realize that the time and space available for a review of such an ambitious subject are limited and， thus， regretfully， we are unable to cover many important contributions.
Conflict of Interest
The authors declare no competing interests.
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