Submicron-Thick Bright White Coatings by Controlling Disorder in Photonic Structures

Nontechnical description Whiteness in conventional paint coatings arises from broadband light scattering across the visible spectrum. However, because light scattering alone is not highly efficient at reflecting light, such coatings are typically thick and often require multiple applications. This project aims to reduce the required coating thickness by approximately a factor of 100 through a fundamental study of optical transport in submicron-thick nanostructured films. In these films, optical transport results from a combination of light scattering and interference, which together dramatically enhance reflectivity beyond what scattering alone can achieve, while also suppressing the iridescent colors typically produced by interference. The proposed submicron-thick bright white coatings will be realized by optimizing the balance between structural order and disorder in the nanostructures, leveraging the interplay between scattering and interference. In addition to advancing knowledge in optical transport, this project will offer important societal benefits through educational components such as visualizations of structural ordering and by significantly reducing the use of white titania pigments—materials that are strategically important to the national economy due to their widespread use and reliance on imported mineral feedstocks. Technical description This project aims to achieve unprecedented submicron-thick bright white coatings through a fundamental study of optical transport in ultra-thin films featuring controlled disorder in nanorod-based photonic structures. In this intermediate regime of order and disorder, both multiple scattering and optical interference govern light propagation. Gaining a deep understanding of how light interacts with such structures in thin films is essential for developing ultra-thin, highly reflective white coatings. To achieve this goal, the research will establish a theoretical framework tailored to these nanostructured systems, perform numerical simulations, and conduct experiments using colloidal fabrication techniques that enable controlled disorder in the position, diameter, and direction of nanorods. The theoretical framework will generate knowledge about (i) how various types of disorder influence the distribution of photonic states across the spectrum, and (ii) how reflectance correlates with film thickness across different regimes of optical transport. Numerical simulations will serve to validate the theoretical predictions. With solid understanding of disorder effects established, the fabrication process will be optimized to achieve unprecedented reflectance throughout the visible spectrum in submicron-thick coatings. Position disorder will be controlled by manipulating the eccentricity of polymer coatings on the nanorods and removing them after self-assembly; diameter disorder by mixing nanorods of different sizes; and direction disorder by tuning magnetic fields applied to the magnetically sensitized nanorods during layer-by-layer assembly. The resulting advances in understanding optical transport and developing fabrication strategies are expected to enable next-generation ultra-thin white coatings with substantial industrial impact. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.