Abstract
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.