Direct observation of micron-sized silica rattles to demonstrate movability of inner spheres in the silica compartment suspended in aqueous media

Micron-sized silica rattle particles were directly observed in aqueous media with an optical microscope to demonstrate the movability of inner silica spheres within the compartment of the silica shell. The rattle particles were prepared by a combined method of fabricating multilayered particles (silica sphere/polystyrene (PSt) inner shell/silica outer shell) and removing the polymer component in the multilayered particles. The polymer component of PSt was removed by heating the multilayered particles at 500 °C for 4 h, which resulted in a successful preparation of silica rattle particles without any residual polymer component. The silica rattle particles in the presence of polyvinylpyrrolidone used as a viscosity enhancer were observed with the optical microscope under an alternating electric field, revealing that less than 10% of inner spheres in the pearl chain of rattle particles could be randomly moved in the compartment of the silica shell. For increasing the percentage of inner spheres randomly moving, the silica rattle particles were slightly etched with a diluted NaOH solution to detach the inner spheres from the inside wall of the silica shell. The slight etching of the silica component led to the observation of approximately 75% inner spheres randomly moving under the applied electric field. This is the first direct observation to show that the single spheres incorporated in the shell compartment are capable to freely move without sticking to the inside wall of the shell.