Bicolored Janus Microparticles Created by Phase Separation in Emulsion Drops

In this work, a simple but versatile method to create bicolored Janus microparticles using phase separation of two immiscible polymers confined in emulsion drops is reported. With a capillary microfluidic device, monodisperse oil drops containing two immiscible polymers and dye molecules are generated in the continuous water phase. As the organic solvent is depleted by evaporation, the drops evolve to Janus microparticles with two compartments. One of the compartments is selectively stained by spontaneous migration of the dye molecules, thereby providing optical anisotropy. In addition, the Janus microparticles can be further rendered to be magneto‐responsive by embedding aligned magnetic nanoparticles in the polymer matrix. Moreover, the configuration of Janus microparticles is exclusively selected from the core‐incomplete shell, dumbbell, and acorn, according to the surfactant in the continuous phase. This phase separation approach will provide a new opportunity for designing Janus microparticles with advanced functionality. Janus microparticles are prepared by phase separation of two immiscible polymers confined in emulsion drops. Spontaneous staining of selected compartment by dyes renders the microparticles bicolored. Magnetic nanoparticles embedded in the polymer matrix provide a net magnetic moment on the microparticles. Janus microparticles with optical and magnetic anisotropy can be potentially used as active color pigments for reflection‐mode displays.