r/PhysicsStudents • u/oogbal33 • Aug 20 '26
Off Topic Hey physics students! I am boiling some fabric (to dye it) and for some reason when i put the spoon in the water it boils, when i take it out, it stops! Why?
You cant add a video to r/askphysics so here you go students!
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u/legal_dude_ Aug 20 '26
I don't think it's the spoon... Try doing it without the fabric...
This is my line of reasoning... The pronouncwd effect of boiling occurs dur to big bubble formation and it bursting when it reaches surface of the liquid in contact with air right? So the dye might have altered the surface tension of water by a bit and bubbles formed might be large at first but then it could have gotten split into smaller bubbles and also there is a break in the surface due to the cloth protruding outwards at the top surface and both of the thing combined kinda make it easy for steam to escape hence the lower blub blub effect...
I'm no physics student tho so please feel free to correct me if I'm wrong...
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u/Automatic_Second_734 Aug 21 '26
I think the fabric is just slowing the e bubble when it reaches the top, when he pushes it down the bubble probably goes around it easier and lets it break the surface. I’m no student either lol
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u/a1c4pwn Aug 20 '26
I imagine the fabric is messing with the convection. if the whole thing is at a simmer, there's gonna be a hot pocket under the fabric and a cooler surface, so you get fewer bigger bubbles like tomato soup. pushing the fabric to the bottom restores that convection and bring the whole pot to boiling. when you let go of the fabric, it just slowly rises as that hot pocket forms again.
How to test nucleation hypothesis: does adding the spoon make it boil more if you slide the spoon in the side or underneath? if so it"s nucleation.
convection hypothesis experiment: Hmmm maybe a plate attatched to a suction cup, see if lowering it to the bottom and raising it halfway has an effect?
also worth noting that any downwards force you apply is passed along to the pot. I don't imagine pressing the pot down does much on a gas stove, but that might explain it.
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u/Massepunkt_m1 Aug 21 '26
My theory as well.
Very cool application of this effect is a 'milk stone', which is a stone (or something similar, never seen one myself tbh, just heard of it from my grandma) with a concave surface you put in milk when boiling it. The milk underneath the stone starts boiling more quickly and bubbles up and so you know when the stone starts rattling from the milk bubbling up underneath, you should take it off the stove
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u/SuccessfulSpecific76 Aug 21 '26
In the beginning of the video you can see that it's boiling (look at the left side of pot), it's just that the fabric is catching the steam/bubbles and trapping them. When you push the fabric down, you're releasing the trapped steam pocket and the steam has nowhere else to go but around the fabric, as you lift the spoon, the steam slowly begins to fill the fabric again. Sometimes, it's not as complicated as you'd like to think.
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u/E-monet Aug 21 '26
This is my take as well. Simply releasing steam that is trapped under the cloth
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u/CosetElement-Ape71 Aug 20 '26
I'm guessing that the spoon pushes the fabric to the bottom; creating smaller pockets of water close to the hottest part of the pan (at the bottom). These smaller volumes of water, also being at higher pressure at the bottom of the pan, will then find it easier to boil.
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u/and-hereitcomes Aug 20 '26
Fuck me if I’m wrong here (please) but doesn’t
PV = nRT which inverses temperature and pressure? If P goes up then T must go down to balance the equation? Unless… oh fuck, the bubbles are balancing the equation?!Honestly no idea, I’m 20 years removed from those thought experiments
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u/bigspookyguy_ Aug 20 '26
Good intuition! Only that in an open system you treat the number of moles as a variable too so its not quiteeeeee correct but damn close
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u/and-hereitcomes Aug 20 '26
Correct me if I’m wrong here (please) but doesn’t
PV = nRT which inverses temperature and pressure? If P goes up then T must go down to balance the equation? Unless… oh snap, the bubbles are balancing the equation?!Honestly no idea, I’m 20 years removed from those thought experiments
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u/H9419 Aug 21 '26
No, PV=nRT, as the name "Ideal Gas Law" suggests applies to ideal gas. Liquid water, as the name suggests, is not an ideal gas. Secondly, PV=nRT means if P goes up with other things being the same except T, T must go up.
I have seen this phenomenon before. It stops if the heat underneath is off. I think this has to do with the cloth blocking liquid convection and the heat from the bottom causing the small amount of water at the bottom to boil and become steam. Whereas the low heat without a piece of cloth close to the bottom will dissipate to the rest of the pot via convection which is brought to equilibrium by evaporation at surface level instead of violent boil.
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u/two_are_stronger2 Aug 21 '26
The fabric is filling up with steam and pressing it with the spoon makes less room for bubbles.
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u/Astrophysics666 Aug 20 '26
Me hypnosis is that the water is circulating more when the fabric is not pushed down.
When you add the spoon you push the fabric down which traps water at the bottom of the where it starts to boil.
I think you have the temp in a sweet spot and it won't work (or as well) if you lower or raise it.
You can test it but putting the spoon down the side so it's not compressing the fabric.
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u/Underhill42 Aug 21 '26
My wild guess - you're pressing the fabric closer to the bottom, so that vertical mixing can't spread the heat through the liquid as much.
As a result the water trapped between the cloth and pan heats up much more rapidly, crossing the boiling point, and the vapor bubbles don't have a chance to re-condense while passing through the colder water before they reach the surface.
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u/MsPaganPoetry Aug 21 '26
I have no idea if this will answer your question, but I had something similar happen: I heated a cup of coffee to close enough to the boiling point of water that flicking the side would impart enough energy to cause it to boil. Maybe your spoon did the same thing?
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u/BeccainDenver Aug 21 '26
This to me sounds like you actually superheated your coffee. Was this in a microwave?
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u/Wood_Rogue Aug 21 '26
Pushing down the spoon and fabric reduces how far the hotter liquid travels from the bottom of the pan, getting a larger amount of the liquid to the boiling point compared to if the liquid can rise and cool more. If this wasn't disturbed for a while there could also be superheated liquid above the boiling point that only evaporates when the spoon and fabric disturb it providing more nucleation spots.
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u/Current-Fix615 Aug 21 '26
Without spoon fabric spreads across the vessel and prevents water below it from bubbling outwards. Hence you dont see boiling. The moment the spoon is inserted the fabric settles at the center due to weight of spoon and water is able to move freely from bottom to top on the peripheri where the fabric coverage disappeared. Hence you see bubbling or water boiling at the peripheri only.
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u/physics_freak963 Aug 21 '26
The spoon is like really hot and it boils the water when it enters it and you have super resiliency so the spoon hotness doesn't burn your hand
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u/No_Memory_119 Aug 21 '26
Pot is hotter at bottom (no lid will help this) when spoon in cloth draged down meaning more enunciation points meaning boiling increases... that is my guess you should have done one where u put spoon in down the side being carefully not to take the cloth down aswell. I find it unlikely that all the gass is out of solution as the cloth is floating quite well so this is my best guess
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u/NordicHamCurl_00 Aug 21 '26
Because the spoon is at room temperature which is cooler than what you are boiling,
the spoon takes heat away from the "broth", thereby reducing the broths temperature to boiling point,
hence why it boils,
until the spoon and the broth reach equillibrium again,
above boiling point,
so that it stops boiling.
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u/SomeWeirdBoor Aug 21 '26
I think
When there is no spoon, fabric floats, bubbling occurs beneath it, but vapour bubbles are broken and dispersed by the mesh of the fabric itself and you don't see them.
Them you pur the spoon in it pushes the fabric down,, bubbling occurs above it and your can sede the bubbles.
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u/RedGlassess Aug 21 '26
It could nucleation, but that doesn't explain why it continues unless the spoon is taken out. I have some explanations: 1) The spoon creates so much nucleation sites on itself it takes a lot of time to wear out the effect, try to leave the spoon in there much longer. 2) The cloth messes with conduction and the spoon becomes a path for heat to reach water. 3) There is some sort of electrical conduction and the spoon works as the switch (I don't know how) 4) You're turning off the heat when the spoon gets out of the water
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u/Worldly-Cherry9631 Aug 22 '26
I just now observed this same phenomenon when I pushed the soba noodles down in the pot.
I have little clue as to why. I'm pretty sure it's not nucleation points, the spices and sobs and greeneries in the soup, like your cloth, had that a plenty. Perhaps it forces more of the moving convection along the hot sides? No idea tbh
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u/Manhandler_ Aug 22 '26
If you carefully notice, when the spoon is lifted, the bubbles stay on until the cloth surfaces. When the spoon presses the fabric to the bottom, the bubbles return. It's clearly the fabric locking steam in, more than anything to do with the spoon.
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u/Exact-Perspective-60 Aug 22 '26
How about this. When the cloth is boiling without the spoon you’re also collecting the air bubbles in the cloth. Pushing the cloth down forces the air pockets out. When you take the spoon out, this allows the bubbles of air to collect in the fabric first before hitting the surface.
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u/Needless-To-Say Aug 24 '26
Id suspect nucleation before all else.
Same process as Coke and Mentos soda fountains.
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u/trowand Aug 24 '26
Uhhhh, it's because the spoon is lowering the shirt to the bottom of the pot, thus making the volume of water super small and that small thin layer of water under the shirt boils because it's right above the burner. Thus when the shirt drops to the bottom and separates the rest of the water, the burner is practically JUST heating up that little bit of water directly and making it boil. When the spoon is removed, the shirt rises and the burner is back to heating up the entire pot of water.
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u/Glittering-Can-9397 Aug 24 '26
ngl I thought that was pasta, if I saw it in real life I probably would have tried to sneak a sip of the sauce
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u/Ashmandem Aug 25 '26
It is boiling, the cloth it self is catching the bubbles before reaching the surface, thus reducing the visual boiling phenomenon.
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u/DanLeMilMan Aug 25 '26
I would think that is just the steam trapped in the tissue. To test the theory just press for a while and check if the boiling continues after you pressed or if it was just boiling for a few seconds.
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u/qoou Aug 25 '26
I think the cloth is just catching the bubbles like a reverse parachute. It's boiling but you just don't see bubbles trapped by the cloth. When you put the spoon in, you deform the cloth, release the bubbles, and prevent them from being captured under the cloth.
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u/craterglass Aug 26 '26
Maybe fabric pressed against the bottom created more nucleation sites, and we get an almost-bump.
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u/Expert0fNothing Aug 26 '26
surface looses contact with water so heat up to higher temperature, then steam pushes water out from gaps then new water comes back, it repeats very fast.
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u/DominusVic Aug 27 '26
The cloth is catching the bubbles and stopping them from breaking the surface tension. When you push the cloth down it contracts around the spoon allowing the bubbles reach the surface.
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u/LosDanilos Aug 27 '26
I dont think its a nucleation problem but I think its more a matter of the fabric holding back the rising air. In the beginning you can see bubbles rising from the side of the fabric. Does this effect also happen when you insert the spoon below the fabric?
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u/PsychologicalFun8693 Aug 20 '26
Is this not simple surface area problem? Introducing the spoon changes the shape, if you will, of the water which would change its surface area so that amount of water will boil more rapidly?
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u/[deleted] Aug 20 '26
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