Long-lived metastable bcc phase in the crystallization of charged colloids under microgravity

Using the reflection spectrometer in Fluid Physics Rack of China Space Station, we investigate the crystallization of charged colloidal suspensions. The colloidal sample, which exhibits a thermodynamically stable face-centered cubic (fcc) phase in normal gravity, manifests a long-lived metastable body-centered cubic (bcc) phase under microgravity. Molecular dynamics simulation on the crystallization of charged colloids is in agreement with the observations of space experiment, as it also shows the formation of metastable bcc crystals. Furthermore, free-energy calculations suggest that bcc-fcc transition cannot be fully realized within the temporal confines of the space experiment. Our findings not only demonstrate the Ostwald's step rule and Alexander-McTague argument specifically for charged colloidal system where the metastable bcc crystals are formed, but also provide further evidence that gravity effects may promote the bcc-fcc transition.