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

Stratification of mixtures in evaporating liquid films occurs only for a range of volume fractions of the smaller component