Abstract
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.
Highlights
Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous poly(methyl methacrylate) (PMMA) films by foaming with CO2 saturation
Being the essential color for interiors, white pigments are widely found in plastics[1], inks and paints[2]
They are frequently used in cosmetics[3] and even food[4]. 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 light[1]
Summary
Inspired by the white beetle of the genus Cyphochilus, we fabricate ultra-thin, porous PMMA films by foaming with CO2 saturation. Cephalopods[9], insects[10,11] and plants[12,13] utilise structural white color[14] for camouflage[9,15], communication and mating[11,16], thermoregulation[17] and pollination promotion[13] Such natural white colors are caused by scattering layers which consist of highly disordered nanostructures. The refractive index contrast between both of these natural materials and the surrounding medium like air is high enough to trigger efficient scattering, even from very thin layers. The famous white beetles of the genus Cyphochilus, for instance, produce their brilliant white appearance with porous chitin scales, which are only 7 ± 1.5 μm thick[24] but effect a reflection of 65–70%21,25. None of the suggested techniques can produce both thin and flexible white film on large scales
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