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.