When a seed is introduced to the randomly moving particles, any particle hitting it will stick to the surface and become part of the seed.
Slowly a growing aggregate will form as more and more particles stick to it. However, instead of the random fuzzy ball of particles maybe expected,
the aggregate will develop a branching pattern often found in nature, for example in lichens growing on a rock, or crystals forming on a surface.
The explanation is that the randomly growing aggregate will develop cavities whose inner surfaces will be hit by fewer particles as they deepen. Diffusion
of particles into them will be limited. Instead filaments will grow, also with occasional cavities, and so on. The aggregate thus develops a self-similar
or fractal character. (The animation stops when the aggregate meets the border. There will be an option for enlagement and printing of the image)
Diffusion-limited aggregation is also at play in the formation of snowflakes. Here certain alignment angles are restricted by the molecular properties
of water but otherwise the crystal forms are random, i.e. infinitely variable.