The scales of the white Cyphochilus beetles are endowed with unusual whiteness arising from the exceptional scattering efficiency of their disordered ultrastructure optimized through millions of years of evolution. Here, a simple, one-step method based on water vapor-induced phase separation (VIPS) is developed to prepare ultra-thin polystyrene (PS) films with similar microstructure and comparable optical performance. A typical biomimetic 3.5 um PS film exhibits a diffuse reflectance of 61% at 500 nm, which translates into a transport mean free path below 1 um. A complete optical characterization through Monte Carlo simulations reveals how such scattering performance arises from the scattering coefficient and scattering anisotropy, whose interplay provides insight into the morphological properties of the material. The potential of bright-white coatings as smart sensors or wearable devices is highlighted using a treated ultra-thin film as a real-time sensor for human exhalation.
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