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Comment by arkitaip | Hacker News Reader

The fact that it can catch microplastics is very impressive.

The article and the press release has no numbers, so it's difficult to estimate how much miroplastic they got. My guess is that the holes in the net are too big to catch microplastic unless it gets stranded with other bigger plastic of plants.

Also, this sentence of the article doesn't make sense:

> The system's success in capturing microplastics came as a welcome surprise since microplastics tend to fall to the ocean floor rather than float on the surface, according to the press release. Since microplasitcs tend to sink, Ocean Cleanup focused on large pieces of plastic.

The plastic float or sink according to it's density, not it's size.

Perhaps larger pieces of plastic contain trapped air that increases their bouyancy.

I'm pretty sure "microplasitcs (sic) tend to sink" is bogus nonsense.

They also float, it really depends on the buoyancy. I think I heard that most of the microplastics are suspending within 5 meters of the surface.

There will be a size below which fluid turbulence determines particle motion rather than particle density. This minimum size will increase for particles with rough surfaces or high shape drag coefficients, or both.

In air, this size is ~ 10µm diameter [1]. In seawater, it should be considerably larger because Reynold's number.

[1] https://www.sciencedirect.com/topics/engineering/airborne-pa...

If it goes to the top or to the bottom is decided by the density. The difference between the density at the top of the ocean and at the bottom is very small 1.020 g/cm3 vs 1.050 g/cm3 . So it's very difficult to find a plastic that is in between https://en.wikipedia.org/wiki/Seawater#Thermophysical_proper...

I agree that the size is important for the viscosity drag. This is the reason why the centrifuges are use in the lab to separate cells of blood or very small precipitates in solution.

This can explain why a sibling comment says that suspending within 5 meters of the surface. If the plastic "wants" to float, but the waves mix the top few meters and it is so small that the drag don't allow it to reach the surface before the next wave mix it again.

But to go to the bottom of the ocean, the plastic needs a higher density.

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