When an emulsion paint is applied to a wall, you are applying a thin film of liquid containing small (mostly less than a micrometre) particles. As the initially liquid film dries the water evaporates leaving these particles behind, and as the water leaves the particles are pushed together, and then these particles coalesce to form the final layer of paint.
I have an active collaboration with my Surrey colleague Joe Keddie to study these systems. I run the modelling side of the collaboration, he runs the experimental side. A postdoc working with us, Andrea Fortini, studied the drying of a mixture of large (shown as red below) and small (yellow) particles. He found that if drying was fast enough, the large and small particles spontaneously segregated into a layer of the small particles on top of a layer of the larger particles (right-hand image). An experimental postdoc, Nacho Martin-Fabiani, then showed this also occurred in experiment, and we published both simulation and experiment in Phys Rev Lett paper (arXiv) in 2016.
The work is described in a news item by the American Physical Society’s news service Physics. It has an embedded movie by Andrea if you want to see the effect happening. There is also a piece on it in the UK’s Institute of Physics’ Physics World. If you prefer the language of Cervantes, you can read the blog post of Francisco Villatoro.
In the Phys. Rev. Lett., Andrea and I also proposed a simple mechanism for the segregation of the small and large particles into layers. But this model neglected gradients in the pressure of the evaporating water. Patrick Warren and I have realised that this neglect is not justified; we have recently published a theory that does consider these gradients (arXiv). Our work also shows that the mechanism is an example of diffusiophoresis — the movement of one species (here the larger particles) in a concentration gradient of another species (the small particles.