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NetLogo Models Library:

Note: If you download the NetLogo application, every model in the Models Library is included. 
This is a 3D version of the 2D model DLA. This model demonstrates diffusionlimited aggregation, in which randomly moving (diffusing) particles stick together (aggregate) to form beautiful treelike branching fractal structures. There are many patterns found in nature that resemble the patterns produced by this model: crystals, coral, fungi, lightning, and so on.
The model begins with an initial green "seed" in the center of the screen. Red particles move around the screen randomly. When a red particle hits a green square, it "sticks" and turns green (and a new red particle is created to keep the process going).
Press SETUP to make the initial seed, then press GO to run the model.
The WIGGLEANGLE slider controls how wiggly the paths the particles follow are. If WIGGLEANGLE is 0, they move in straight lines. If WIGGLEANGLE is 360, they move in a totally random direction at each time step.
The MAXPARTICLES slider controls how many red particles can exist at the same time.
Both settings may be altered in the middle of a model run.
The USEWHOLEWORLD? switch controls whether the red particles start at the edge of the screen, or from just outside a sphere enclosing the green area. If the switch is on, it's easier to see what's going on, but the model runs slower, particularly when WIGGLEANGLE is high.
Note that the resulting structure has a branching structure, like a tree. Why does this happen?
What other phenomena in the world do the shapes remind you of? Is this aggregation process a plausible model of how those phenomena occur?
When the enclosing sphere gets too near to the edge of the screen, the model stops, since allowing the particles to wrap around the edges of the screen would distort the shape of the aggregate.
New red particles are created not at the edge of the screen, but at the edge of a sphere enclosing the current size of the green aggregate, instead of traveling from the edge of the screen. Also, if a red particle wanders too far outside the sphere, it disappears and a new one is created. Neither of these behaviors is essential to the model  it is done this way just to the model runs fast.
Try different settings for WIGGLEANGLE. What is the effect on the appearance of the resulting aggregate? Why?
Does the MAXPARTICLES slider make any difference? Why or why not?
Do you think the USEWHOLEWORLD? setting has an effect on the appearance of the resulting aggregate? Why or why not? Experiment and find out. If you initially thought differently from what you found, why do you think you thought otherwise? Can you explain why it does happen the way you found?
What happens if you start with more than one "seed" patch? What happens if the seed is a line instead of a point?
Can you find a way to modify the code so the resulting pattern spirals out instead of radiating straight out?
The rule used in this model is that a particle "sticks" if any of the eight patches surrounding it are green. What do the resulting structures look like if you use a different rule (for example, only testing the single patch ahead, or using neighbors6
instead of neighbors
)?
Can you compute the fractal dimension of the aggregate?
If instead of using green, you gradually vary the color of deposited particles over time, you can see more vividly the accretion of "layers" over time. (The effect is also visually pleasing.)
The model will run faster if the turtles are invisible, so you may want to add a switch that hides them (using the hideturtle
command).
Note the use of the neighbors
and distancexyz
primitives.
The various models in the "Fractals" subsection of the "Mathematics" section of the Models Library demonstrate some other ways of "growing" fractal structures.
The "Percolation" model in the "Earth Science" section produces patterns resembling the patterns in this model.
The concept of diffusion limited aggregation was invented by T.A. Witten and L.M. Sander in 1981. Tamas Viczek's book "Fractal Growth Phenomena" contains a discussion, as do many other books about fractals.
If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.
For the model itself:
Please cite the NetLogo software as:
Copyright 2006 Uri Wilensky.
This work is licensed under the Creative Commons AttributionNonCommercialShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/byncsa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.
This is a 3D version of the 2D model DLA.
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