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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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[–] evasive_chimpanzee@lemmy.world 5 points 1 week ago

You might be applying more initial speed, but let's assume that you aren't.

The total delta_V (area under the acceleration-time curve) is the same in both cases. Placing padding under the jar squashes the curve so the maximum force isnt as high, but the impulse is longer.

Tomato paste is thixotropic, so it gets thinner under shear stress. At a certain threshold of shear stress, it essentially becomes liquid.

My guess is the quick spike ends up with less time above that threshold, so less time behaving as a liquid.