  When a liquid film of colloidal solution consisting of particles of different
sizes is dried on a substrate, the colloids often stratify, where smaller
colloids are laid upon larger colloids. This phenomenon is counter intuitive
because larger colloids which have smaller diffusion constant are expected to
remain near the surface during the drying process, leaving the layer of larger
colloids on top of smaller colloids. Here we show that the phenomenon is caused
by the interaction between the colloids, and can be explained by the diffusion
model which accounts for the interaction between the colloids. By studying the
evolution equation both numerically and analytically, we derive the condition
at which the stratified structures are obtained.
