Uniform Coating of Self-Assembled Noniridescent Colloidal Nanostructures using the Marangoni Effect and Polymers

Colloidal crystals exhibit structural color without any color pigment due to the crystals' periodic nanostructure, which can interfere with visible light. This crystal structure is iridescent as the resulting color changes with the viewing or illumination angle, which limits its use for printing or displays. To eliminate the iridescent property, it is important to make the packing of the colloidal nanoparticles disordered. Here, we introduce a drop-casting method where a droplet of a water-ethanol mixture containing monodisperse polymer-coated silica nanoparticles creates a relatively uniform and noniridescent deposit after the droplet evaporates completely on a heated substrate. The uniformity is caused by a thermal Marangoni flow and fast evaporation effects due to the heated substrate, whereas noniridescence is the outcome of short-range-ordered packing of nanoparticles by depletion attraction and friction effects produced by polymer brushes. We show that the colors of the final deposits from individual droplets remain unchanged while the viewing angle is varied under ambient light. We expect the coating method to be compatible with ink-jet printing and the uniformly coated self-assembled noniridescent nanostructures have potential for color displays using reflection mode and other optical devices.