Clustering of charged colloidal particles in the microgravity environment of space.

We conducted a charge-charge clustering experiment of positively and negatively charged colloidal particles in aqueous media under a microgravity environment at the International Space Station. A special setup was used to mix the colloid particles in microgravity and then these structures were immobilized in gel cured using ultraviolet (UV) light. The samples returned to the ground were observed by optical microscopy. The space sample of polystyrene particles with a specific gravity ρ (=1.05) close to the medium had an average association number of ~50% larger than the ground control and better structural symmetry. The effect of electrostatic interactions on the clustering was also confirmed for titania particles (ρ ~ 3), whose association structures were only possible in the microgravity environment without any sedimentation they generally suffer on the ground. This study suggests that even slight sedimentation and convection on the ground significantly affect the structure formation of colloids. Knowledge from this study will help us to develop a model which will be used to design photonic materials and better drugs.