Optical properties of water absorbing textiles for camouflage

Abstract: Background matching, a form of camouflage, involves species developing patterns and coloration that closely resemble their environment. Humans have utilized camouflage and background matching, particularly in military applications, to reduce detectable characteristics, known as signatures. Recent advancements in sensor systems necessitate the development of effective camouflage in the short-wave infrared (SWIR) range (0.9 – 2.5 µm). However, absorption of SWIR radiation is heavily influenced by water, and dry textile materials may be easily detectable against a forest background due to their low water content. To address this challenge, this master's thesis explores the integration of hydration onto a textile fabric to reduce the signature in SWIR. Various fabric types were included in this study. The optical properties of these textiles, both when wet and dry, were evaluated using SWIR imaging and UV-VIS-NIR spectroscopy, and compared to foliage. Surface modifications were employed to introduce hydrophobic properties to the fabric, such as the application of water-repelling agents (Nikwax and OrganoTex) or functionalized silica nanoparticles. The water evaporation rates of untreated and surface-treated fabrics were assessed. However, the hydrophobic surface did not significantly reduce water evaporation from the fabrics. Similarly, the addition of a nanocellulose-based hydrogel on the fabric surface did not result in a significant change in evaporation. Combining the hydrogel with water-repelling solutions in a multilayer configuration indicated prolonged evaporation, but further tests are required to validate this finding. This master's thesis demonstrates that introducing water into a dry textile fabric alters its spectral properties, making it more similar to foliage. However, the primary challenge lies in retaining water within the material for improved camouflage in SWIR wavelengths.