  I model the drying of a liquid film containing small and big colloid
particles. Fortini et al. [A. Fortini et al, Phys. Rev. Lett. 116, 118301
(2016)] studied these films with both computer simulation and experiment. They
found that at the end of drying the mixture had stratified with a layer of the
smaller particles on top of the big particles. I develop a simple model for
this process. The model has two ingredients: arrest of the diffusion of the
particles at high density, and diffusiophoretic motion of the big particles due
to gradients in the concentration of the small particles. The model predicts
that stratification only occurs over a range of initial concentrations of the
smaller colloidal species. At concentrations that are either too low or too
high, the concentration gradients due to drying are not enough to push the big
particles away and so produce a layer at the top of only small particles. In
agreement with earlier work, the model also predicts that large Peclet numbers
for drying are needed to see stratification.
