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!

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Two dimensional nanogrids from fast evaporation

This work, published in ACS Nano, is the result of a collaboration between teams of scientists of the University of Surrey and of the University of Southhampton. I was responsible for the modelling and simulation of the process, together with Dr. Richard Sear. Therefore, in this post, I will concentrate on the modelling of the dynamics, and refer you to the main article for details on the assembly process and optical properties.

The system consists of a suspension of large colloidal particles and small gold nanoparticles in water. We decided to model the dynamics of the system via a Langevin Dynamics, where solvent particles are not explicitly included in the simulations. Their most important effects are incorporated via a random force, which gives Brownian motion, and a drag force on the particles.

The evaporation of the solvent is modelled simply by a downward movement of a soft interface at a constant velocity.  In the movie below, the camera follows the downward moving interface. The full system is shown on the left. On the right, we show the same system but with the large particles made invisible.

The large particles accumulate at the top and form a crystalline structure (similar to natural opals) and the small nanoparticles move to the top wriggling through the empty spaces.  The large particles can be seen as a sieve through which the nanogrid, as shown on the right side of the above movie.

The optical properties of the resulting material can be tuned by changing the parameters of the simple and fast assembly process.

Look here for the full story.

Interestingly, the dynamical behaviour of the system changes dramatically when the number of small particles is very large…but that’s a story for another behaviour of the system changes dramatically when the number of small particles is very large…but that’s a story for another behaviour of the system changes dramatically when the number of small particles is very large…but that’s a story for another post and another article.