Recall in the previous post, we simulated droplet crystallization. In the simulation, we formed droplets, and then turned on a flux of . The droplets immediately formed shells of , which we do not observe in experiments. We had concluded in the previous meeting that it would be interesting to see what happens if we allow atoms to diffuse through the droplet at a fast rate.

First, I refined the notions of “extended species”. We define the extended species to be a has exactly neighbors of type , and similarly for . In fact, the code has been modified to handle species of the form , where is a vector of counts, e.g. is the number of Gallium neighbors. But as a first step, we forget about the other counts besides so that, for example is more or less identical to . That is, we obtain the extended type definition above from this more general setting. For now, I used the same bonding energies after we map to the coarser definitions for extended type.

Next, we define the swapping rate of two atoms as

where as before is a constant prefactor, are the local energies of and , respectively. The *diffusion coefficient* , is given by:

- if one, but not both of the atoms is a vacuum (surface diffusion);
- if one of the atoms is and the other is (fast diffusion through droplet);
- otherwise.

I varied . I started with a hemispherical droplet of radius 32 lattice sites. I fixed . I used deposition rates monolayer/second. and . For each I ran two trials. First I simulated 0.1 seconds, measuring the configuration every 0.001 seconds. Second I simulated 5 seconds measuring every 0.05 seconds.

Here are the movies, with each row corresponding to .

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[…] is a follow up of this post. We had observed that, in the movies, once an atom came close to the droplet/crystal interface, […]

Pingback by Droplet Crystallization – Part 3 « Kris's Research Notes — October 19, 2010 @ 11:32 pm