Wednesday, 16 September 2015

Diffusion Limited Aggregation and Brownian Trees

Diffusion limited aggregation (DLA) is a type of stochastic process in which particles move following a Brownian motion until they stick together with other particles.

Imagine a square in which we have put some glue on the bottom edge. Then, we release one particle at a time on that square. The particles follow a random-walk within the square until they stick to the bottom edge or to a particle already fixed or rooted. Then, that particle becomes itself attached to the fixed structure. This fixed structure so formed is commonly called Brownian trees, because it recalls the shape of a tree.

Many natural phenomena, such as electrodeposition, dielectric breakdown (lightings), coral growth, snowflake formation and other not so natural such as urban areas growth can be simulated using DLA. To the basic diffusion process, we can add other physical features such as gravitation, external forces, attraction between particles, fluids, different frames where particles can be attached... to achieve different results.

The snippet of code below is an example of a simple two dimensional DLA simulation written in Python.

The following images were created using the script above, for different type of frames.

On the blog, we can find interesting examples of such processes. There are even several programming libraries to simulate such processes. As usual, adding a litte bit of imagination we can easily hack the script above to generate new wonderful images.

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