Colloidal Self-Assembly and Space Experiments

Abstract

Colloidal particles spontaneously assemble in dispersions to form various ordered structures, including colloidal crystals and regular clusters. The self-assembly of colloids has been actively studied as a model for phase transitions in atomic and molecular systems. Colloidal crystals are one of the ‘photonic crystals’ whose refractive index changes with the structural period of light wavelengths. In particular, the diamond lattice of colloidal particles is a ‘perfect photonic crystal’ with a full band gap for incident light from all directions. The formation of the diamond lattice and tetrahedral clusters, the structural units of the diamond lattice, have been actively studied. Particles with a high refractive index are useful for optical materials. However, the high refractive index particles often have a high specific gravity, which causes them to settle in the medium and this can affect their association behavior. Microgravity environments, where the sedimentation is safely negligible, are useful for experiments on colloidal systems. In 2020, in collaboration with JAXA and many other relevant organizations, we experimented on the self-assembly of charged colloidal particles on the Japanese Experiment Module Kibo of the International Space Station. Experimental results on the number of aggregates and structural symmetry suggested that slight settling and convection on the ground have a significant effect on the association of colloids. We expect that the methods and experimental systems used in this space experiment will be useful also for future space experiments on various kinds of soft-matter.