When a liquid film of colloidal solution consisting of particles of differentsizes is dried on a substrate, the colloids often stratify, where smallercolloids are laid upon larger colloids. This phenomenon is counter intuitivebecause larger colloids which have smaller diffusion constant are expected toremain near the surface during the drying process, leaving the layer of largercolloids on top of smaller colloids. Here we show that the phenomenon is causedby the interaction between the colloids, and can be explained by the diffusionmodel which accounts for the interaction between the colloids. By studying theevolution equation both numerically and analytically, we derive the conditionat which the stratified structures are obtained.
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