Generation of uniform photonic balls and colloidal clusters in soft-microfluidic devices

Abstract

Three-dimensional (3D) colloidal assemblies of sub-micrometer monodisperse latex particles have received much attention recently, because of their potential uses as photonic crystals and photonic molecules. If the shape and size of spherical photonic crystals are well controlled, it can be used in highly reflective paints, light diffusing materials or in electronic paper. And colloidal clusters are applicable to building blocks for novel photonic crystal, sensor technologies and microphotonics because of their morphology dependent resonances (MDRs). In this paper, the fabrication of such uniform colloidal aggregates using soft-microfluidic devices will be discussed. In soft-microfluidic devices, uniform emulsions were generated at regular intervals by droplet break-off at the junction of two microfluidic channels. The balance of surface tension and high shear forces at the leading edge of the water that is perpendicular to the emerging oil flow determines the diameter of droplets and generation frequency. Droplet size is increased as surface tension and flow rate ratio of aqueous phase and oil phase are increased. And polystyrene suspension whose mass concentration is less than 30% is used as aqueous phase, but there is no significant change in flow pattern such a low mass concentration. Generated droplets suspended in oil kept moving along the microchannel and was shrunk down slowly as water was dissolved into the oil phase and if PS suspension solution with adequate concentration is used as aqueous phase, consolidated spherical colloidal aggregates were obtained at the downstream of the microchannel. For high concentration of suspension solution, the number fluctuation was negligibly small and within a few percentages. For low concentration, however, the number fluctuation of colloidal aggregates became significant and the generated colloidal aggregates were not uniform.