Synthesis of the solar heat reflecting membrane by duplicating the Saharan silver ant micro-hair structure

This project’s goal was to mimic the Saharan silver ant’s micro-hair arrays by creating a solar heat reflecting membrane with parallel-aligned Al-ZnO microrods using a Mayer rod coating method. The synthesis of Al-ZnO microrods was carried out using the hydrothermal method. The membrane was then produced by mixing the microrods with liquid silicone rubber and spreading the mixture into the shape of a membrane using a Mayer rod. The dragging of the Al-ZnO microrod, confined between the Mayer rod and the polypropylene film, induces the partial alignment of Al-ZnO microrods in the dragging direction. We note that the distribution of microrod alignment is well described by the Lorentzian function and that the full width at half maximum is measured to be 22.3°. Most Al-ZnO microrods are encased within silicone rubber. At wavelengths between 200 and 1100 nm, the membrane blocks light almost entirely. The solar heat gain coefficient of our solar heat reflecting membrane is 75% lower than the membrane containing commercial ZnO powder.


Introduction
Extreme heat severely seriously affects comfort and increases cooling loads. One of the main sources of heat in the tropics is solar heat [1]. In the past year, the leading journal "Science" has published an investigation into the novel cooling mechanism of the Saharan silver ant (SSAnt), which allows it to survive the scorching heat of the desert [1][2][3]. The SSAnt has an array of parallel-aligned microhairs made of low refractive index chitin [3] in contrast to standard artificial heat shielding coatings, which contain high refractive index pigments like titania or reflective metals like aluminum. However, the micro-hair improves the SSAnt's solar heat shielding ability by (1) acting as an array of optical prisms to reflect most of the incident solar heat away from the body and (2) increasing the thermal emissivity for effective body heat disposal via radiation [1][2][3]. This structure is also flexible, light, and breathable, making it a great passive cooling technique for buildings and summer clothing. Although this discovery sparked the development of a coating with high optical reflectivity and thermal emissivity [4][5][6][7][8], there are only a few follow-up researches about the SSAnt micro-hair structure [9][10][11][12]. This is because the aligned triangular micro-hair with the downward oriented face being smooth and the other two ones being rough is too intricate to be replicated. To simplify the situation, we use parallel-aligned Al-ZnO microrods within silicone rubber as the artificial SSAnt micro-hair structure and compare its solar heat shielding ability with a commercial ZnO powder which is nodular in shape [13].
There are various techniques employed to align the nanowires, including the Langmuir-Blodgett technique [14], shape memory polymer shrinking [15], capillary printing with nanochannels [16], water-bath assisted convective assembly [17], meniscus-dragging deposition [18], and nano-template imprint-lithography technique [19]. These techniques successfully align the nanowires but require complex preparation such as additional transfer processing, pretreatment of the substrate, or prepatterning at the nanoscale, and cannot be applied in large-scale applications. It has recently been discovered that nanowire alignment can be achieved via a simple Mayer rod coating process that is highly suitable for large-scale applications [20,21].

3
By creating a solar thermal reflecting membrane with parallel-aligned Al-ZnO microrods using the Mayer rod coating process, we were able to replicate the SSAnt micro-hair network in this project. We then investigated whether the aligned Al-ZnO microrod provided superior solar heat shielding ability to the commercial ZnO powder. Synthesis of the microrods was carried out via the hydrothermal method. The membrane was then produced by mixing the microrods with liquid silicone rubber and spreading the mixture into the shape of a membrane using a Mayer rod. The microrod size was measured via SEM and compared to that of the SSAnt micro-hair. Al-ZnO microrod membrane was examined using optical and scanning electron microscopes to make sure that microrods were properly aligned within the membrane. The solar heat shielding ability was also investigated by measuring UV/ Vis spectra and the solar heat gain coefficient.

Synthesis of Al-ZnO microrods via sol-gel method
The synthesis of Al-ZnO microrods was carried out by the wet chemical method modified from our previous method [10,22], as shown in Fig. 1. A clear solution of zinc ions was first prepared by dissolving 0.037 M of the zinc precursor, 0.00185 mol of aluminum isopropoxide, and then 8 M of NaOH in 100 ml of deionized water. Second, a nonionic surfactant solution was prepared by dissolving 30 ml of Triton X-100 into 900 ml of deionized water. 117.8 ml of zinc ion solution and 882.2 ml Triton X-100 solution were mixed in a polypropylene beaker. The mixture was cured in a hot-air oven (Binder). The curing temperature was set to 80-100 °C and the curing time was set to 20 h. The beaker was covered with a polyethylene film to reduce water evaporation. The solution was then allowed to cool naturally to room temperature. The as-prepared white precipitate was then washed with deionized water followed by ethanol and centrifuged at 5000 rpm for 3 min multiple times. The resulting microrods were finally air-dried at 100 °C for 4 h.

Fabrication of bionic coating agent
During the preparation of the coating agent, 5 g of Al-ZnO microrods was placed in 7 ml of silicone oil. They were mixed with vigorous stirring and were then dispersed with an ultrasonic disperser to form an Al-ZnO microrod/silicone oil dispersion. The mass ratio of Al-ZnO microrods within the dispersion and the density of the dispersion are calculated to be 0.417 and 1.47 kg/m 3 , assuming that the Al-ZnO microrod has the same density as ZnO. The coating agent was then prepared by mixing Al-ZnO microrod/silicone oil dispersion, ELASTOSIL ® LR 6200 A, and ELASTOSIL ® LR 6200 B in the mass ratio of 2:5:5. The filler ratio and the Fig. 1 The synthesis process of the Al-ZnO microrods 1 3 density of the coating agent are calculated to be 0.069 and 1.27 kg/m 3 . The coating agent's formula was chosen with care. The microrod would be difficult to disperse and would aggregate if the filler content was too high. The number of microrods in the microscopic image would be too few for an appropriate determination of microrod alignment, though, if the filler content was too low. The coating agent was finally placed in a vacuum chamber to remove trapped bubbles. It is stable for about 5 days before use.

Fabrication of solar heat reflecting membrane
As shown in Fig. 2, the solar heat reflecting membrane was prepared by the Mayer rod method. The bionic coating agent was sprayed on a cleaned polypropylene film during the coating process. A Mayer rod was used to spread the bionic embedding agent along the spacers (length: 23 cm, wire diameters are 100 µm) at speeds around 1 m/min. The membrane thickness was adjusted using two spacers with 0.4 mm thicknesses. To limit the interaction between microrods and substrate during the Mayer rod coating process, it is crucial to provide sufficient thickness. After thermal curing at 100 °C for 20 min, the sample was allowed to cool naturally. The resulting membrane was finally carefully peeled off the polypropylene film.
Silicone membrane and ZnO powder membrane were produced in the same procedure for comparison, except that the silicone membrane did not contain any microrods and the ZnO powder membrane contained commercial ZnO powder instead of Al-ZnO microrod.

Characterization
Scanning electron microscopy (SEM) (Leica Stereoscan 440, 20 kV) was used to observe the sample morphology. Samples were placed on carbon tape and then sputter coated with gold for approximately 2 min. The elemental composite was measured via energy-dispersive X-ray spectroscopy (EDX). The size distribution of the Al-ZnO microrod is measured with the software "ImageJ." The alignment of Al-ZnO microrods within the membrane was measured using the Nikon Optiphot POL and was analyzed with the software package OrientationJ of the software "ImageJ." The ultraviolet-visible (UV/Vis) spectra were obtained using the Hitachi Double Beam Spectrophotometer UH5300. 10 mm Quartz spectrometer cuvettes were used. For powder samples, 5 mg of the sample was dispersed into 3 ml deionized water with the assistance of ultrasonic. For film samples, a small sample in the size of 10 mm × 20 mm was cut and attached to the cuvette surface. Horizontal attenuated total reflection-Fourier transform infrared spectroscopy (HATR-FTIR) was performed on a PerkinElmer Spectrum 100 FTIR with a HATR accessory. ZnSe crystal window was used. The data was collected over 4 scans with a resolution of 4 cm −1 .
The solar heat gain coefficients (SHGC) of the SSAnt membrane were measured to evaluate their solar heat shielding abilities. The simulated sunlight (Newport, AM 1.5 G, Class ABB) was used as the light source and the incident light power was measured with a solar power meter (PMKIT-21-01 Newport, 0.19-10.6 µm wavelength range). SHGC is defined as the fraction of incident solar radiation transmitted through the membrane, both directly transmitted and absorbed and subsequently released inward [10]. Expressed as a number between 0 and 1, it is computed with the formula:

Results and discussions
The micro-morphology of the Al-ZnO microrod was analyzed via SEM as shown in Fig. 3. Its hexagonal cross section differs from that of SSAnt micro-hair. Triangular hairs offer the best reflectivity enhancement, although hairs in other shapes can also offer reflectivity enhancement, according to the simulation results of Shi et al. [1]. It suggests that our Al-ZnO microrod should be able to improve solar heat shielding.
Its lengths are 10-40 μm and its diameters are 1.9-2.8 μm. To compare the size of the Al-ZnO microrod with that of SSAnt micro-hair, the size distribution of SSAnt micro-hair (1) SHGC = transmitted solar heat total incoming solar heat was analyzed by using the SEM image of the head of the SSAnt from the paper by Shi et al. [1] which shows that the length of Saharan silver ant micro-hair is 10-80 μm and its diameter is 1.9-2.8 μm. The Al-ZnO microrod asprepared has a similar diameter as the SSAnt micro-hair while its length is comparable to that of the shortest SSAnt micro-hair.
According to the EDX data, the weight percentages of zinc and aluminum in the microrod are 64.47% and 0.39%, respectively. In other words, just a small amount of aluminum was doped onto the microrod during the production process, and microrods are primarily ZnO.
The UV-Vis spectra of the Al-ZnO microrod and commercial ZnO powder are presented in Fig. 4. The spectra were normalized to improve the comparison of their shapes and to remove the effect of concentration fluctuation brought on by the instability of the microrod/powder inside the aqueous dispersion. Commercial ZnO powder has a well-defined absorption band at 366 nm. Its absorbance is relatively high in the UV range, but its absorbance decreases significantly with wavelength. In contrast, the Al-ZnO microrod has no well-defined absorption bands. The absorbance increases slightly with increasing wavelength. These results demonstrate that the Al-ZnO microrod has a higher shielding ability in the near-IR range than commercial ZnO powders. The high absorbance of the Al-ZnO microrod in the near-IR range is due to two factors. Firstly, the morphology of the sample has a large effect on its UV-Vis spectra. Based on Attia et al., rod shape particle has higher reflectivity at a high wavelength range while sphere shape particle has higher reflectivity at a low wavelength range [23]. As a result, the rod shape structure of the Al-ZnO microrods improves its near-IR shielding ability. Secondly, doping Al into ZnO can improve its opacity in the high wavelength range [24]. Figure 5 shows the photograph of the solar heat reflecting membrane. The membrane is white and soft, and its thickness is 0.3 mm. It exhibits homogeneity inside a membrane at the macroscopic level and has a consistent tint. No  mass decrease would take place during the coating process because ELASTOSIL ® LR 6200 is additive liquid silicone rubber and the curing temperature is substantially lower than the boiling temperatures of the coating agent's constituents. The coating should have a similar filler ratio and density to the coating agent, i.e., 0.0694 and 1.27 kg/m 3 correspondingly, if there is no considerable volume change during the curing process. The membrane's mass per square meter is estimated to be 382 g/m 2 . Given that ZnO makes up the majority of microrods, the ZnO membrane should have a filler ratio and mass per unit area similar to the microrod coating. Figure 6 shows its optical microscopic image. The majority of the Al-ZnO microrods are evenly distributed throughout the coating, but some of them have clumped together. The alignment distribution of the microrod within the solar heat reflecting membrane is shown in Fig. 7. The alignment distribution can be well described with the Lorentzian function. The full width at half maximum is measured to be 22.3°, implying that the Al-ZnO microrods are well aligned within the solar heat reflecting membrane. The SEM image of the solar heat reflecting membrane is presented in Fig. 8. It shows that the membrane has a smooth surface. The microrod (the light white bars in the SEM image) is well aligned. This agrees with the optical microscopy image and shows that the parallel alignment of Al-ZnO microrods within the membrane is achieved. The density of microrods in the SEM image is lower than that in the optical microscopic image because most microrods are embedded within the membrane and cannot be observed.
Al-ZnO microrods were aligned using capillary printing by sliding Mayer's rod over a bionic embedding agent onto polypropylene film under constant speed and pressure. A schematic of the preparation of the solar thermal reflective membrane using this capillary printing technique is shown in Fig. 9. In this work, we utilized a Mayer rod (length: 23 cm, wire diameter: 100 μm) to create the microchannels inducing the alignment of the Al-ZnO microrods in the printing direction. The Mayer rod was in contact with the substrate during the dragging of the bionic coating agent. This facilitated the uniform formation of air-liquid-solid meniscus lines behind the contact points between the stamp and substrate [16,20,21]. Droplets of the Al-ZnO microrods solution are deposited on the polypropylene film and soaked into the line-patterned Mayer rod. Next, the dragging of the Al-ZnO microrods, confined between the Mayer rod and the polypropylene film, induces the partial alignment of Al-ZnO microrods in the dragging direction.
The FTIR spectra of the solar heat reflecting membrane and silicone membrane are shown in Fig. 10. Both   [24]. The absorption band at 1422 cm −1 is due to the unreacted vinyl groups [24]. There is no obvious difference between the FTIR spectra of the solar heat reflecting membrane and the silicone membrane. This is because most Al-ZnO microrods are embedded within the silicone coating.
The UV/Vis spectra of the solar heat reflecting membrane and silicone membrane are shown in Fig. 11. The solar heat reflecting membrane almost completely blocks light in the wavelength range of 200-1100 nm. In contrast, the silicone membrane can only completely block light in the 200 to 400 nm wavelength range.
The solar heat gain coefficient (SHGC) of the solar heat reflecting membrane, ZnO powder membrane, and silicone membrane is shown in Fig. 12. The SHGC of the solar heat reflecting membrane is 0.055, 91% lower than that of the silicone membrane (0.621) and 73% lower than that of the ZnO powder membrane (0.205). The excellent optical opacity of solar heat reflecting membrane is the main reason for its excellent solar heat shielding ability. The outcome supports our earlier research and demonstrates that aligned Al-ZnO microrods offer greater solar heat shielding ability [9][10][11][12].

Conclusion
We have successfully duplicated the SSAnt micro-hair network in this work by synthesizing a solar thermal reflecting membrane containing parallel-aligned Al-ZnO microrods. The length of the Al-ZnO microrod synthesized via the hydrothermal method is about 10-40 μm and its diameter is around 1.9-2.8 μm, comparable to those of SSAnt micro-hair. Compared to the commercial ZnO powders, the Al-ZnO microrod is more effective at blocking near-IR. A Mayer rod was used to synthesize the solar heat reflecting membrane, in which Al-ZnO microrods are parallelly aligned in the coating along the coating direction. The alignment distribution is well described with the Lorentzian function and the full width at half maximum is measured to be 22.3°. Most Al-ZnO microrods are embedded with silicone rubber. The SSAnt membrane almost completely blocks light in the wavelength range of 200-1100 nm. The solar heat gain coefficient of solar heat reflecting membrane is 73% lower than that of membrane synthesized with commercial ZnO powder. Availability of data and materials Not applicable.

Declarations
Ethical approval Not applicable.

Competing interests
The authors declare no competing interests.