practical applications of white surfaces.
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Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous PMMA films by foaming with CO2 saturation. Optimising pore diameter and fraction in terms of broad-band reflectance results in very thin films with exceptional whiteness. Already films with 60 µm-thick scattering layer feature a whiteness with a reflectance of 90%. Even 9 µm thin scattering layers appear white with a reflectance above 57%. The transport mean free path in the artificial films is between 3.5 µm and 4 µm being close to the evolutionary optimised natural prototype. The bio-inspired white films do not lose their whiteness during further shaping, allowing for various applications.
White is the most popular color for today’s industrial products1. Being the essential color for interiors, white pigments are widely found in plastics1, inks and paints2. Additionally, they are frequently used in cosmetics3 and even food4. In many of these products, their whiteness is achieved by incorporating titanium dioxide (TiO2) particles which, due to their high refractive index (RI) of n ≈ 2.6, effectively scatter incoming visible light1. Fabricating white polymeric films and bulk parts with TiO2 particles causes some issues: (i) an extra operational step that increases costs, (ii) complicated disposal procedures due to possible environmental harm by TiO2 particles, (iii) pulmonary inflammation effects5,6, and (iv) suspected health issues7. Therefore, particle-free alternatives are of particular interest8.