This is the Diffusion-Limited Aggregation cellular automaton model. Particles (blue) diffuse around in the grid, i.e. move randomly. When a blue particles touchs a green solid particle, it also turns green and "sticks" (aggregates), i.e. stops moving. A branching structure grows over time. The shape of the branching structure depends on the initial density of blue particles.
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Controls specific to this particular applet are: