Simulation Of Transpiration And Solar Spectral Reflection Characteristics Of Natural Leaf

Thermal infrared imaging is an important detection technique that can distinguish subtle distinctions in the infrared radiation temperature of the targets and their background.Considering that vegetation is one of the typical backgrounds,it is effective to simulate the thermal infrared temperature of natural leaves with membrane bionic materials composed of hydrophilic ingredients countering the thermal infrared detection.In addition,hyperspectral technology is becoming more mature,and the development trend of camouflage technology is to develop bionic materials with solar spectrum and infrared compatible camouflage functions.Considering that vegetation is one of the most common backgrounds,a bionic material which can simulate the solar spectral reflectance and infrared thermal characteristics of plant leaves was developed,and the bionic mechanisms were analyzed with experiments and theoretical models.The cooling effect of transpiration can significantly affect the daytime infrared radiation characteristics of leaves.In order to investigate the transpiration characteristics of plant leaves,the stomata distribution characteristics of various plant leaves were tested,and the water evaporation characteristics of isolated stomata and stomata arrays were analyzed using the finite element method.It was found that the edge diffusion effect of the spindly micron-scale stomata and the stomata distribution patterns approaching the extremal flux line are the reasons for the high-efficiency diffusion characteristics of the stomata array of the natural leaves.Considering that temperature and humidity are the important environmental factors that affect the transpiration process of the leaves,the stomatal resistance of various plant leaves was tested in Wuxi in summer and winter and Xishuangbanna in summer to obtain the transpiration rate of the leaves.A heat and mass transfer was constructed and the heat dissipation characteristics of plant leaves were analyzed.The analysis results of the radiative heat transfer,convective heat transfer and transpiration heat transfer process of the leaves in the three environments show that in summer and winter in Wuxi,the transpirative heat flow is about 55.8%and 24.3%of the total heat dissipation,respectively.It shows that in summer with high temperature environment,transpiration heat transfer has a significant effect on plant leaf temperature,while in winter with low temperature environment,the effect of transpiration heat transfer on the leaf temperature will be greatly reduced.In Xishuangbanna summer,the transpirative heat flow is about 37.3%of the total heat dissipation,indicating that environmental humidity can also significantly affect the effect degree of transpiration heat transfer on the leaf temperature.Based on the research on the transpiration process of the leaves,a bionic material with hydrophilic materials PVA and LiCl and Cr2O3 particles was prepared to simulate the cooling effect of transpiration of the leaves.In order to study the moisture sorption and desorption characteristics of the biomimetic material,the kinetics characteristics of the sorption components of the total moisture uptake were analyzed with a triple-mode sorption model.The results show that the process of pooling adsorption occurs only when the amount of Henry absorption exceeds a threshold.The relative humidity corresponding to this threshold is named as the critical humidity of the sorption kinetics curve.When the environmental humidity is below the critical point,no pooling adsorption occurs in the film,and the total moisture uptake is extremely low.When the critical point is exceeded,the amount of pooling adsorption gradually increases,and the total moisture uptake increases abruptly.With the increase of LiCl content,the threshold of Henry absorption and the corresponding critical humidity gradually decrease,indicating that introducing LiCl in the membrane mainly affects the process of pooling adsorption.According to the radiation,convection and moisture sorption process of the bionic material in the natural environment,a transient heat and mass transfer model was constructed and the heat transfer characteristics of the bionic material in typical summer weather days in different regions were analyzed.The results show that during the daytime,the temperature of the bionic material can be regulated through the moisture desorption process,indicating that the cooling effect of transpiration of the leaves can be simulated by the bionic material.The humidity during the night and the difference in humidity between day and night are both the key factors affecting the amount of moisture desorption.In order to realize the compatible camouflage performance of the biomimetic materials,the mechanism of solar reflectance characteristics of the leaves was studied with PROSPECT model,and the reasons of the symmetry of solar reflectance curve and its physiological significance were clarified.Based on the results,two types of biomimetic materials are prepared.One type utilizes Cr2O3 particles to simultaneously simulate the absorption characteristics in the visible light band and the scattering characteristics in the solar spectrum band,and is named as COG-BM.The other type contains chlorophyll and TiO2 particles to simulate the absorption and scattering characteristics in the above two bands respectively,and is named as CH/TD-BM.The measured results show that both can simulate the solar spectrum reflection characteristics of the leaves approximately.In order to optimize the bionic performance,the reflectance predicting model was constructed with Lorenz-Mie scattering and four-flux theories.On this basis,the reflectivity prediction model of biomimetic materials was established by using the four-optical flow theory,and the parameters of the reflection characteristics of biomimetic materials were analyzed to optimize the reflection characteristics.In order to deal with the threat of hyperspectral detection in different seasons,it is necessary to further consider the preparation of bionic materials under the background of withered yellow leaves.Two kinds of membranaceous yellow biomimetic materials were prepared by dispersing the particles of chrome titanium yellow and iron oxide yellow as fillers in polyvinyl alcohol films respectively.Reflectance and transmittance of the biomimetic materials were measured,and absorption and scattering coefficients of the biomimetic materials were inverted with a four-flux model.Results indicate that the biomimetic material adopting chrome titanium yellow particles can simulate the solar spectrum reflection characteristics of yellow leaf because of the similar absorption and scattering characteristics.The biomimetic material adopting iron oxide yellow particles cannot simulate the spectrum reflection characteristics of yellow leaf near the wavelength of 900 nm due to the characteristic absorption of the iron oxide.When the volume fraction of the chrome titanium yellow particles is lower than 2.12%,the absorption and scattering coefficients both increase approximately linearly with the increasing volume fraction,indicating that the particles can scatter radiation independently.Therefore,the reflectance of the biomimetic material can be regulated through linearly changing of the volume fraction of the chrome titanium yellow particles.