Optical properties of photonic crystal balls of different sizes synthesized using a microfluidic device

In this study, a microfluidic device was used to synthesize photonic crystal balls (PCBs, i.e., spherical colloidal crystals) from polystyrene colloid solutions. Water-in-oil (W/O) droplets containing monodispersed polystyrene colloids were formed using a T-junction microfluidic device. By controlling the flow rate of the continuous oil phase, the droplet diameters could be adjusted between 220 and 410 μm. The droplets had excellent monodispersity, with a coefficient of variation (C_v) in size of less than 2 %. By carefully evaporating water from the droplets, PCBs with vivid structural colors and photonic bandgaps (PBGs) were obtained at visible wavelengths. By increasing the diameter of the polystyrene colloids used to construct the PCBs, the PBG wavelengths along the [111], [220], and [222] directions were red-shifted, which was consistent with the modified Bragg's law. The spherical shape of the photonic crystal spheres caused the PBGs to exhibit almost no dependence on the angle of light incidence, exhibiting consistent backward scattering of structural colors at all angles of light incidence (unlike polystyrene colloidal crystal thin films, for which a strong dependence of the PBG on the incident angle of light was observed). These results provide valuable insights into the optical properties of PCBs.