Self-assembled structures of submicron-sized colloidal particles are anticipated to serve as innovative optical materials, including photonic crystals. Colloidal particles with high refractive indices, which are beneficial for optical applications, often have a high specific gravity. During the self assembly process on the ground, they experience gravitational sedimentation. Consequently, a microgravity environment is ideal for studying their structural formation. Our group has participated in colloidal crystallization and cluster formation experiments aboard the International Space Station. In the space experiment, we utilized titania particles (TiO2, refractive index ≃ 2.4, specific gravity ≃ 3) with a diameter of approximately one micrometer. Here, we report the construction of two-dimensional (2D) colloidal crystals of the titania particles on the Earth, by applying the knowledge of particle fabrication obtained during the preparation of the space experiment. Furthermore, we present a preliminary study on stacking the 2D crystal layers of the titania particles. These results will be valuable for constructing photonic crystals on the Earth and for future space experiments on colloidal self-assembly.

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This study reports on the phase separation and solidification behavior of Fe–Cu alloys under microgravity, utilizing an electrostatic levitation furnace (ELF) aboard the International Space Station (ISS). Spherical Fe–Cu samples of varying compositions were first prepared on Earth using an aerodynamic levitator in an argon atmosphere, then selected samples were melted and resolidified in space without a container. Detailed thermal histories were obtained, revealing distinct cooling plateaus and recalescence events, indicative of complex solidification dynamics. Microstructural analysis by electron probe microanalysis showed the formation of a core shell structure, with Fe-rich phases forming the inner regions and Cu-rich phases surrounding them, as well as evidence of iron oxide formation likely due to residual oxygen. The cooling rates and resulting morphologies were found to agree well with phase field simulations, suggesting that slower cooling in microgravity promotes the development of well-defined core-shell structures through enhanced droplet movement and mixing. These findings provide new insights into phase separation and solidification mechanisms in Fe–Cu alloys under microgravity and will be compared with results from other compositions in future work.

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The periodic oscillation of diffusion flame, known as (bulk) flame flickering, is a well-known dynamic behavior connected with flow-associated instabilities triggered by gravity-induced buoyant flow. In this study, the flickering behavior of a wick assisted flame was experimentally examined under partial gravity fields ranging from 1 G to 0.075 G, where G is the normal gravity acceleration (9.81 m/s2). The subjected gravity was controlled using the “Slope-Sliding-Method environment 2,” which allows repeatable and finely adjustable gravitational conditions in laboratory experiments. Temporal variations in flame height were recorded and analyzed using fast Fourier transform to extract characteristic frequencies. The specific objectives of this work are (1) to determine the gravity dependence on flickering frequency and (2) to observe the “critical” gravity, where flickering is suppressed under a substantially reduced gravity regime. The results indicate that the well-known Strouhal–Froude number correlation is preserved down to ~0.17 G, while distinctively different flickering behavior occurs below this point owing to a different mode of periodic oscillation. Theoretical analysis indicates that the characteristic delay time by the conductive heat transfer in the present wick-assisted flame system plays a role under such highly reduced gravity conditions. Although special care must be taken to investigate the “critical” gravity, this work is the first to reveal that weak flickering is preserved, even under a Lunar or Martian gravity environment.

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