Preparation of structurally colored, monodisperse spherical assemblies composed of black and white colloidal particles using a micro-flow-focusing device

It is very annoying when colors fade over time from exposure to bright light. A structurally colored material is a strong candidate for the fabrication of non-fading colored materials composed of environmentally friendly and non-toxic chemicals. We demonstrate that by using a micro-flow-focusing device, monodisperse spherical assemblies displaying various structural colors in air can be prepared from a suspension containing environmentally friendly white and black colloidal particles: monodisperse submicron-sized SiO2 colloidal particles and black magnetite colloidal particles. The average size of the monodisperse spherical assemblies can be controlled by changing the amount of SiO2 colloidal particles used in the preparation of the suspension. The hue of the monodisperse spherical assemblies can be varied by simply altering the size of the SiO2 colloidal particles. These monodisperse spherical assemblies exhibit either iridescent or non-iridescent structural colors, depending on the aggregation state of the SiO2 colloidal particles, which can be controlled by the presence or absence of an electrolyte. Moreover, the saturation of the colors produced by the monodisperse spherical assemblies can be altered by modifying the amount of magnetite colloidal particles that is added. Our results should offer new possibilities for structurally colored materials for future application in pigments and test agents.