Stratification in dynamics of sedimenting colloidal platelet-sphere mixtures
Dynamics of sedimentation in binary mixture of colloidal platelet-sphere was theoretically studied by using the minimal energy model. Revisiting such sedimenting mixture of the platelets, we find that the nematic phase behaviors are formed coupling with the certain stratification structures in the mixed sediments, which has never discussed before. The non-equilibrium sedimentation-diffusion equations involving the excluded volume interactions between the colloids have been developed to show the occurrence of the stratified structures in the mixed platelets-spheres sediments. The model presents clearly how the nematic configuration occurs corresponding to the stratifications (the peak-like concentration profiles) over time. Both of initial spheres concentration and the size ratio are found to have great effects on two coupled resulting structures, which has been illustrated by presenting two state diagrams in details. It is found that the stratification structure of the sphere-on-top corresponds to the nematic bottom phase configuration for small size ratio, meanwhile, the platelet-on-top structure occurs corresponding to the floating nematic phase configuration for large size ratio. Interestingly, existing the moderate range of size ratio (qualified by the borders), the mixture only displays the nematic phase configuration (either nematic bottom phase or floating nematic phase depending on the sphere loadings), while the mixed sediments are unstratified as a bulk.