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When a drop of ink or watercolour paint is placed on wet paper, rather than uniformly diffusing in all directions it forms tendrils which rapidly expand out from the spot.

Why is this and what is the chemistry or physics occuring?

This video may further show the type of thing I am referring to

https://motionarray.com/stock-video/black-silver-tendrils-pack-216423

It appears that the pigment solvent forms discrete channels outwards through the water. I wondered if this might be a surface tension interaction or something else.

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  • $\begingroup$ If you look closely at paper, you will see that the paper itself is not uniform. $\endgroup$ – Jon Custer Nov 5 '20 at 16:37
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If you’re talking about a gentle placement of the drop on the paper, I’d guess it’s likely due to the fibrous structure of the paper providing “easier” paths for the ink to flow. If you’re talking about an impacting drop, the drop will vibrate when it impacts the paper until the forces of surface tension are exceeded and the drop “splatters”.

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