Infrared Optical and Thermal Properties of Microstructures – from Nature to Bio-inspired Materials
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Infrared Optical and Thermal Properties of Microstructures – from Nature to Bio-inspired Materials

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Control over mid-infrared optical properties is critical for applications such as thermoregulation, infrared sensing, thermal imaging, and thermal camouflage. The distinct wavelength ranges for heating and cooling by radiative heat transfer call for selective emitters to fulfill the need for spectral optical control. The present doctoral research focuses on the role played by microstructures in controlling the infrared optical properties of materials and understanding the relationship to temperature and impact on thermal properties. There is a plethora of examples of microstructure-dependent optical properties in nature. The present work focuses on the mid-infrared optical and thermal properties of microstructures and aims to learn from nature in developing bio-inspired microstructures geared towards varying human needs and applications.Butterfly wings have garnered much attention towards studies on their visible optical properties, but not much is known about their infrared optical and thermal properties. The mid-infrared wavelengths of 7.5-14 μm coincide with the atmospheric transmission window and are particularly important for radiative heat transfer in the ambient environment. A high mid-infrared emissivity can allow for cooling by aiding in heat loss to outer space, whereas a low mid-infrared emissivity can minimize this heat loss. This work uses Fourier-transform infrared (FTIR) spectroscopy and infrared thermography to show that the mid-infrared emissivity of butterfly wings from warmer climates is up to 2 times higher than that of butterfly wings from cooler climates. I also use rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) computations to reproduce the spectroscopy data. The spectral emissivity values govern the thermal properties of butterfly wings, and the thermal computations show that butterfly wings in their respective habitats can maintain a moderate and viable temperature range through a balance of solar absorption and infrared emission. Using the computed optical properties, a potential correlation between the mid-infrared emissivity of their wings and climatic conditions such as air temperature has been investigated. In a similar vein, Saharan silver ants make use of their hair-like microstructures to enhance their mid-infrared emissivity and remain cool in the desert climates. This work replicates the relatively simple hair-like structures in the form of corrugated, periodic microstructures to control the infrared optical properties of various materials. Furthermore, I add tunability to the optical properties by utilizing elastomeric substrates and show mid-infrared emissivity control by mechanically reconfigurable graphene. Here, mechanical stretching and releasing induce controlled morphology changes. By integrating graphene with stretchable, elastomeric substrates, I fabricate 10 µm pitch crumpled graphene optimized for mid-infrared emissivity control. The interference between adjacent crumpled features and diffraction at the graphene/air interface leads to wavelength-specific variations in transmissivity, and consequently emissivity for the crumpled graphene at 9.9 µm and 13 µm wavelengths. The measurements also show reversible changes of emissivity over 30 cycles of stretching and releasing at the wavelength of 9.9 µm. To present a more practical system, I have developed a selective emitter using a simple metallic coating. By evaporating nickel on a pre-strained elastomer, I create 700 nm periodic corrugations that increase the nickel absorptivity from 0.3 to 0.7 in 0.2-2.5 µm wavelengths due to multiple scattering, as supported by spectroscopy and computations. The optical change is reversible and is used to demonstrate a stretchable wearable system. The corrugated nickel on a human body at 309 K allows a heat flux of 62 Wm-2 out of the skin when stretched and 79 Wm-2 into the skin when released. The present doctoral research thus studies the role of microstructures in the optical and thermal properties of natural beings and applies the findings towards bio-inspired materials that aim to satisfy ever-growing human needs.

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