Void-Free Photonic Surfaces created by Adaptive Dense Packing of Emulsion Droplets

Abstract

Nature has created periodic nanostructures confined in microcompartments and assembled them to construct macroscopic photonic surfaces. Inspired by nature, we have designed photonic surfaces through a secondary assembly based on adaptive dense packing of emulsion droplets. The emulsion droplets are composed of silica particles dispersed in photocurable resin at which the particles spontaneously crystallize because particles repel each other. Upon evaporation of the surrounding fluorocarbon oil, the droplets densely pack to attain the shape of hexagonal disks without coalescence, resulting in a void-free coating. Interestingly, the shape transformation of droplets causes the colloidal crystals to align along their flattened surfaces, which results in structural colors with high brightness, saturation, and angle dependence upon specular reflection. The randomly oriented colloidal crystals within the droplets render the coatings noniridescent upon backward reflection. This secondary assembly strategy enables the production of mosaic monolayers or different-colored multilayers for color mixing through simultaneous or consecutive assembly of distinct droplets. The low surface tension and high volatility of fluorocarbon oil facilitate the deposition and rapid packing of droplets, thereby ensuring that this adaptive assembly is applicable for various target surfaces of two-dimensional and three-dimensional objects. Therefore, this technique potentially presents a general means to create structural color coatings.