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Ever since using less oil in my skillets I've been noticing how it (or molten grease) always concentrates around the exterior, leaving the centre dry. Annoyingly that's also always the hottest area, even on my induction hub.
This happens regardless of the skillet I use, also with my heavy cast-iron ones that definitely don't have warped bottoms.
What's the explanation for this? Is it simply the bottom doming a tiny bit because the centre heats/expands most and "concaving" isn't an option (my copper sautée pan does just that btw)? Or is there also something going on in the liquid fat?
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12-21-2023 02:14 PM
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As a lapsed physicist and keen cook, I can't explain what you're seeing. In fact, I've noticed that, if I use just a small amount of oil in my iron skillet, it will stay in the centre of the pan unless I explicitly swill it around. This is my favourite science-based cook - maybe worth messaging him in case he has an explanation for you: @ChrisYoungCooks
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I'll assume that your skillet isn't warped so will just ask what kind of heat source you use.
Originally Posted by CliffR
I do seem to recall that this didn't happen on the gas burners of my mom's stove (several of her cast alu skillets actually warped to concave so that figures) nor on the old-style cast-iron burners I had in my Parisian appartment. I'd be willing to believe those made for more homogenous heating, but I didn't have one matching my large 12" skillet ...
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Probably like you say. The expansion at the outer edges is more restricted because of the structural integrity of the curving surfaces. Towards the center it is just flat and expanding into the stronger perimeter where the curved edges are. Thus the action is to push everything away from the center even if it is only pushing heated oil. My guess anyway.
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Maybe you're cooking "repulsive" food?
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Not a physicist, but a damn good cook, if I might say so myself...
My guess is the "wall" of the skillet is simply providing an easier place for oil to get trapped. Couple that with the fact the food (if you're say, blowing chicken breasts) isn't touching those out edges as much as it is the center, therefore the oil there is not sticking to/absorbing into the food.
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Hmmm, the liquid itself will expand too of course, and do so more in the hotter centre. Since you can't compact or "stretch" a liquid that means a puddle will end up pushing itself away from the centre, in whatever direction it had the furthest outlier? Originally Posted by lammie200
NB: this also happens without any other food in the skillet; food can even provide a barrier to the effect.
There's an old Dutch book with a whole collection of similar little problems, called "Natuurkunde van het vrije veld" (physics of the field, field here referring to the prairie/pasture kind). The one at hand would be a perfect fit, I feel.
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My skillets aren't leaky so no, the oil doesn't get out of the pan
I'm not going to put a video on YT but the grease definitely pulls away from the centre. Not systematically always to the same side so it's not a question of the glasstop being tilted.
I don't scrub my CI skillet either. Years of proper treatment have given them a healthy layer of seasoning that makes them no-stick for most things, so all they need is some hot water and the few caresses with the special rag I have for this purpose. Any unwanted, living proteins that remain or develop get burned off during the preheating stage next time I use them
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No, not particularly. I can't even say that the centre is used most. The very outskirts have of course seen less frequent use but I don't see what kind of mag(net)ic attraction they could have when I put a oil in the centre.
I will admit that I usually do put swirl the pan to get an even oil coating all over the business surface. An evenness which never persists as per the original question. I'll have a look what happens when I just put a puddle of oil in the centre.
EDIT: this is basically also why I put a new skillet upside down in the oven when I'm putting a first few layers of seasoning on it: that avoids "caking" in places where seasoning oil agglomerates.
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I never said it goes up the side walls?! Originally Posted by ragman1
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Yep. That’s the way I see it. Originally Posted by RJVB
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The bottoms of skillets are seldom actually flat. Some are concave, some are convex, almost none are perfectly flat. We have skillets and other pans with both types of bottoms, and I don't know what causes either, other than the imperfect manufacturing process.
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Gas. Originally Posted by RJVB
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What is this barbarism? How do people live without induction hobs?
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Nobody ever said 'now we're cooking with induction hobs'
Crap physics hypothesis time. Assuming the heat source is directly under the centre of the pan (which it isn't, but neither are cows spherical), then we can expect a heat gradient high at the centre to lower at the edge. Any liquid in the pan is therefore also likely to be somewhat hotter at the point near the centre than the point further from it. This would create higher pressure at the centre, leading to a force propelling it away from the centre.
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I guess I'd have to take a good fret rocker to the next new skillet I get then. I agree that there will always be some margin but judging from the fit to a glasstop that was far from a no-brand entry-level model mine are pretty close to flat on the outside. Two of them are premium brand, non-cast steel, one with a sandwiched alu bottom, the other thick carbon steel formed in a bad-ass press. Originally Posted by sgosnell
I do suppose one can wonder modern manufacturing techniques actually aim to make the bottoms almost unmeasurably convex (seen from the business side) as a way of insuring the pans won't become concave and wobble on electric hubs. A slight convex warp is probably also less problematic from an energy transmission viewpoint.
Re: induction: it was considered when we prepared our moving 4 years ago. I bought a small, cheap mobile 2000W induction hub for testing as a much earlier experience had been very underwhelming. I'm still using that thing regularly because it'll get water to cook more quickly and is more energy-efficient when using small pots. But it was clear very quickly that we have a number of incompatible pans and that those "adapter disks" don't work well enough (they cause the hub to overheat). Gas is expensive here and my partner has a phobia against it so we went with halogen again.
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I'm bored with this now. The question's been answered and is just common sense. We're not talking about old, wonky frying pans that have lost their shape, it's about cast-iron skillets that have no reason to be concave at all.
Also, if the OP's using less oil than usual then a high central heat will make the oil very fluid in the central area and more viscous at the edges. The food itself will absorb some of the oil leaving a peripheral ring outside that. There may well be some sort of science that compels the viscous oil to move away from the centre but I'm not smart enough for that.
I would have thought the worst effect would be the danger of food at the centre sticking and burning. Answer: reduce heat and use more oil. Or use a larger gas ring. Or pre-heat the skillet, etc, etc.
Personally, I use a wok and never have a problem at all unless I'm careless. And I'm not careless. I like my stir-frys!
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Originally Posted by ragman1
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I struggle to understand the kind of mind that states they're bored of a thread but feels the need to tell everybody about it and then contribute further ignorant statements about the same.
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Gone fishing.
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In that case I think this is what you want. Now you'll be happy. Except he used a non-stick pan.
Physicists finally figured out why food sticks to a frying pan
Why does cooking oil sometimes suddenly retreat from the centre of a pan, leaving dry patches that cause food to stick? Scientists have pinpointed the physics behind this, and their findings could help to refine some industrial processes.
Alexander Fedorchenko at the Czech Academy of Sciences and a colleague recorded experiments with an overhead camera focused on a commercial non-stick pan with a coating of sunflower oil 1.5 millimetres thick. They discovered that uneven heating caused a process known as thermocapillary convection, which can draw oil towards the edges of a pan.
In their tests, the oil heated up faster in the centre of the pan. Surface tension tends to decrease in liquids as temperatures rise, which led to a gradient in surface tension across the pan. The stronger tension toward the edges pulled the less cohesive oil at the centre outwards and deformed the surface of the layer.
At this point a rupture can occur in the thinner part and leave a dry spot. The team found that beyond a critical size, the hole in the oil can’t heal. This critical point differs by temperature for each liquid, but is surpassed when the curved upper surface of a liquid is ruptured by the pull from the outer edge of the pan and surface tension is lost.
Physicists finally figured out why food sticks to a frying pan | New Scientist
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Non stick pans are the worst. Get a metal pan and you can soak and scrub it.
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Thanks. Non-stick coating just makes the phenomenon more easy to observe (and the polymerised fatty layer that "seasoning" is made of shares some of the properties of teflon and the like). Originally Posted by ragman1
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Here's a YT channel -
Uncle Scott's Kitchen - YouTube
He tests CI, SS and carbon steel pans - - very well - - and mentions often how pans with less thermal mass are more easily affected ( adversely ) by induction burners due to the heat concentration.
In one vid he speaks about high-buck CS pans becoming ' spinners' due to center warping of the pans under induction heat. He's also mentioned how some manufacturers are ' designing in ' an offset, so that under heat, the pan bottom expands to level. One recent test showed a CS pan going from a non-spinner when cold to a spinner under heat, and then back to stable when cooled.
Good luck !!!
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Seems interesting, thanks! Originally Posted by Dennis D
(My favourite cooking YT channel is that of "Cowboy Kent Rollins" ... for a slightly different kind of "high-tech" but lots of cast-iron!
As I mentioned above, my (Mauviel) copper sautée pan easily becomes a "spinner" when I heat it too quickly, even on my relatively slow halogen burners. The workaround is to preheat it with maybe 1cm of water which acts as a buffer. When it boils I know the pan is hot enough so I dump the water and pour in some oil (in the centre because the remaining water is gone there first )
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