SimuLab 11: Building an Aggregate by Computer

You need the Aggregation program to complete this SimuLab. You can download it to your computer or use it online in a Java applet.	Download Aggregation:
			Java
	Mac OS 7.x	Windows 95/98/NT/2000

Can the aggregate building process be made faster? Of course! Let the computer make random choices and move the walker. The computer moves the walker much faster than we can, but uses the same simple process.

Open the Aggregation Kit program. On the right is a black screen with a so-called "seed'' molecule in the center.

1. The settings on the left regulate the process by which the seed will grow. For now, make these settings:
• Upper slider: Speed. Set to a position near the left: 10 steps/sec.

• Cell size: Set to a middle position.

• Particle movement buttons: Select Random.

• Fill gaps: Slider to the left - Off


2. Press Go (upper left button).


A random-walking "molecule'' is released far away from the seed and staggers back and forth, up and down, until it finally touches and sticks to the seed. Immediately a second random walker is released, ultimately touching and sticking to the pair at the center. Then the third, fourth, fifth, hundredth walker is added to the growing pattern. In order to speed up the process, if the walker is too far from the pattern, the computer does not continue the random walk step by step. Instead, it places the particle at a point nearer the seed, as if it reached this nearer point by a random walk.


3. Now try new settings: faster speed, smaller cell size.


4. Find settings for which the resulting pattern looks like the result of your electrodeposition experiment. When you grow such a pattern, choose Save under the File menu, using a name you can remember. You will need this saved pattern later.

Q4.14: Why is the DLA pattern so "leggy,'' so "spidery?'' Why isn't it a compact blob? In your answer, think of the growth process in detail: the walker staggering back and forth as it passes inward between the ends of two legs.

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