Watching paint dry: stratification by size

In my previous post, I have discussed the formation of nanogrids via solvent evaporation. This occurs in mixtures of large and small particles at low concentrations of small particles.

At large concentrations of small particles, we find a new physical effect that separates  particles by size during solvent evaporation.

The solvent evaporation is a quite general process and used in many industries. Among these, of course, the paint industry. The exciting stratification effect that we found while watching paint dry made the news…and a collection of articles can be seen here in Altmetric. The scientific article that combines both simulation and experimental results was published in Physical Review Letters. This was a collaboration between teams at the University of Surrey and at the Université Claude Bernard, Lyon.

The model we have used to describe the paints is similar to the one discussed in the nanogrids post; a mixture of large and small particles that move according to the Langevin dynamics, i.e. they have  Brownian motion and feel drag forces.

The evaporation pushes the particles towards the bottom substrate and, if the right conditions are met, the small particles push the large ones away!


In the movie above, I show the evolution of the system during evaporation as seen in our computer simulations. The left side shows a lateral view of the entire simulation box. Its height is 1500 times the diameter of the small particles. The total number of particles in the system is 72000, and in this case, there are 152 small particles for each large one. To the right, we show the region close to the top air-water interface.  The camera follows the interface downwards movement. Indeed, the small particles push away the large ones, leading to a final stratified film.

This result was really surprising and we made sure to understand the mechanism behind it before publishing the results. To thie end, we developed a simple model that captures the physics behind stratification. I will explain this model in a future post. For now enjoy watching paint dry!