Preparation of uniform colloidal crystal spheres by using an electrohydrodynamic spray

Abstract

Colloidal assembled structures have been of special interest due to their characteristics such as photonic band gaps, diffraction patterns, high catalytic throughput and high packing density. Colloidal assemblies have been patterned and mounted for useful objects as themselves or used as a sacrificial template to develop porous structures. Among the various kinds of mounted shapes, a sphere may represent the simplest form driven by the minimization of interfacial free energy, which can be realized by using droplets. In this experiment, we prepared suspension droplets with narrow size distribution that contain silica, hydrophobic silica, and polystyrene beads respectively in cone-jet mode by electrohydrodynamic atomization, and subsequently spherical colloidal crystals, called photonic balls, by evaporation-induced self-assembly inside the droplets. The size of the suspension droplets was easily controlled by the AC field strength and the flow rate. The silica photonic balls were prepared in the size range of 10 - 40 ㎛ with bimodal size distribution. The SEM images of a photonic ball fractured revealed the random arrangement at the center part. The photonic balls redispersed in deionized water showed the characteristic color, green, corresponding to the size of building block beads, 252 nm, due to the opaline structure in the vicinity of the surface of a ball. On the other hand, organic photonic balls had a broad size distribution and showed the hollow ball shape with a rough surface. They were tinged with milky color due to their irregular surface. AC field enabled the photonic balls to be mass-produced due to its charge-diminishing property, while DC field led to extremely low yield owing to deposit of suspension droplets into the electrode. The production rate of the silica photonic ball under AC field was estimated at 34.25 mg/hr. And it can be increased as one intends by adding capillaries in the electrospray system. Our photonic balls can be applicable to ...