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I was doing some experiments with water drops on lampblack when I saw this.

enter image description here

You can see the full video here. Inside a water drop which is perfectly still from the outside, you can see some moving things, which I suppose are some lampblack flakes from the surface. I am not bothered about how those flakes got in there, but I am interested in their motion.

Why are they even moving? Is it some kind of Brownian motion?

I am amazed by the dynamicity inside such a seemingly inactive water droplet.

If I had not seen this, I would have considered the water drop as a simple sphere for the rest of my life. This observation, at least for me, raises an important question.

Is anything in the world really NOT dynamic?

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Based on the similar movement of inhomogeneities on soap bubbles, I'd say the flakes' movement is caused by air currents around the drop and perhaps thermal convection.

enter image description here
Source: https://youtu.be/LM3p3X92mWI

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    $\begingroup$ +1 Yes, I have confirmed. The speed of motion of the flakes increases when I blow on them. $\endgroup$ – Krishnanand J Feb 15 at 4:31
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Those are flakes of lampblack. Their wettability in water is unequal around their periphery. This allows surface tension forces to apply a netforce in some random direction on the flake and force it to scoot around, until the wettability difference goes away as the water begins to wet out the previously poorly-wetting spots.

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    $\begingroup$ But if the flakes were independently subjected to random forces, they wouldn't move cohesively as they do in the animation. I believe, as I propose in my answer, that the flakes are being driven either by air currents around the drop of by convection in the drop. $\endgroup$ – stafusa Feb 15 at 1:27
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    $\begingroup$ sounds like an experiment needs to be performed! $\endgroup$ – niels nielsen Feb 15 at 5:22
  • $\begingroup$ Indeed. Now there's a new piece of evidence: the OP commented in my answer that blowing over the drop make the flakes move faster. $\endgroup$ – stafusa Feb 15 at 9:11
  • $\begingroup$ very cool! if there is no air movement, is there any residual particle movement at all? keep me posted- NN $\endgroup$ – niels nielsen Feb 15 at 13:44
  • $\begingroup$ @Gert - where does the energy come from to do this? $\endgroup$ – TLW Feb 15 at 19:59

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