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(Note: another similar question from a few years ago yielded nought but speculation. I have at provided some detailed observations in the hope that the community can come up with something rigorous as an explanation...)

We disassembled an electronic product and found this interesting pattern of dust on the plastic (ABS, HDPE or similar material)...

enter image description here

Observations:

  1. The dust is easily wiped off.
  2. There is no cooling fan in the product and it was situated in an unventilated cupboard. Any airflow would be a result of thermal convection for the most part.
  3. It's an old broadband/DSL router which ran from a 12V wall wart ("double insulated" so no Earth-ground connection).
  4. The patterns seem to congregate around some of the minor injection-moulding features (known as "ejector marks", the 4 small circular features are almost co-planar with the main surface surrounding them, they are extremely shallow and have a slightly different surface finish to the rest of the plastic).
  5. The appearance of the features ("lightning" springs to mind) suggests to my mind that this is some sort of electromagnetic / electrostatic effect.
  6. The plastic is not coated with any kind of electrically conductive EMC coating.
  7. The circuitry housed within doesn't seem to indicate any particular correlation between the placement of electronic components and the locations of the dust patterns. The features of the plastic case seem to be a more significant catalyst for the formations.

Questions:

  • How do these interesting patterns form?
  • What is the composition of the dust likely to be (e.g. metallic or something else?).
  • Why does the shape of the plastic seem to catalyse these weird shapes?
  • Why the lightning / fractalline appearance?
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    $\begingroup$ Possible duplicate of Reason for strange "canyons" of dust on the cooling platform $\endgroup$ – John Rennie May 9 '17 at 17:42
  • $\begingroup$ @JohnRennie, well I'd say the phenomenon in question is the same for sure, but that other thread has no convincing answers with evidence or citations. I'll put a bounty on this in due course. $\endgroup$ – Wossname May 9 '17 at 18:18
  • $\begingroup$ Likely the different surface finish also indicates a different ability to store up static charge. Then the dust will start gathering along the field lines, and results in features that can charge up as well, leading to the dendritic pattern you see. $\endgroup$ – Jon Custer May 9 '17 at 18:39
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    $\begingroup$ @JohnRennie I agree, very similar to previous post. But looking at that, I don't believe that it was ever answered correctly. I suspect this might be a charge accumulating around the mold features that's attracting the dust. But why such a fractal pattern? I vote to let it stand. $\endgroup$ – docscience May 9 '17 at 19:23
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    $\begingroup$ Also I suggest to let it open. I bookmark this question - if it gets an acceptable answer, this and the dupe candidate will be mergeable. $\endgroup$ – peterh - Reinstate Monica May 9 '17 at 19:51
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I would suspect that either the plastic or the dust is statically charged. The charge interacts with the surroundings and will settle in the most stable position available. Once one particle is in place it adds it's charge to the system affecting how the next dust particle falls and so on. The patterns are the from the cumulative effect of the falling particle charges. As for where they start, the potential differential of the ridges or seams in the plastic probably attracted them.

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    $\begingroup$ Intuition is good and can lead to some useful new avenues of investigation but I'd like to find some scientifically sound underlying principles for this phenomenon. Who knows, it might lead to some remarkable new revolution in efficiency for the injection moulding industry... or not. I want to steer away from answers with "probably" or "I suspect" because they are speculative and don't lead to strong conclusions necessarily. $\endgroup$ – Wossname May 9 '17 at 19:27
  • $\begingroup$ @Wossname looks like Lindberg may have a possible underlying principle with reference. $\endgroup$ – docscience May 10 '17 at 0:56
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Perhaps the molten, extruding plastic carries with it a charge separation which 'freezes' in place when the plastic cools in the mold. Trapped "electret" charges then follow a pattern resembling plastic flow patterns.

https://en.wikipedia.org/wiki/Thermo-dielectric_effect

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