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why does a thick, viscous material settle better when I knock the container against a wet rag than when I hit the counter against a harder object? (lemmy.blahaj.zone)
submitted 2 weeks ago by dandelion@lemmy.blahaj.zone to c/askscience@lemmy.world
14 comments fedilink hide all child comments
 

let's say I have spooned some tomato paste into a glass jar, and there are all these pockets of air and I want to collect the tomato paste at the bottom of the container.

The tomato paste is viscous / thick enough that gravity is not causing it to settle anytime soon, so if I pick up and then slam the jar down, the forces can cause the paste to push down and settle into the jar.

I notice if I slam the jar on a cutting board or a hard counter-top, the tomato paste doesn't settle very much. When I slam the jar on a rag, it seems like it settles better.

If I had to guess, I'm curtailing some of the force when hitting hard surfaces because I'm worried about the glass breaking, and I don't like the noise so I hold back to reduce noise.

A folded-over damp rag however is softer and cushions the jar, so it doesn't make noise and it feels like I can get away with using more force - so maybe more force = better settling?

I'm tired, so I thought I would ask why the rag trick works - but now I think I understand.

Anyway, let me know if you disagree with my reasoning here, or if I've overlooked something.

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[–] dandelion@lemmy.blahaj.zone 4 points 2 weeks ago* (last edited 2 weeks ago) (2 children)

hm, I don't think so - non-Newtonian fluids are like water + cornstarch: they flow like a liquid unless you apply force, then they harden into a solid.

The tomato paste is just not a liquid, it's so thick it functions like a solid. So if you spoon some of it into a jar, the paste will just stay where you spooned it, it won't settle like a liquid would. AFAIK, a non-Newtonian fluid would settle like a liquid when not under force, unlike the tomato paste.

[–] threelonmusketeers@sh.itjust.works 2 points 1 week ago

they flow like a liquid unless you apply force, then they harden into a solid.

Not all non-Newtonian fluids are like this. Some are reversed: They flow like a liquid when pressure is applied, and harden into a solid (or gel) when the pressure is released. I think ketchup is one such example.