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I have a Ceran cooking field at home, it's a glass-ceramic surface with heating coils embedded beneath. When I slide a pan around there's much more friction on a hot plate than on a cool one. Why?

Later clarification: My cooking field has regular thermal heating, it's not an induction oven.

The oven:

oven

The pan, notice the concentric grooves:

pan

The pan has clearly seen better days, maybe the organic residues do play a role, as John Rennie suggested.

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    $\begingroup$ Hi Christian. Just to clarify, are you saying that the friction between the Ceran and the metal pan increases with temperature i.e. the pan slides less easily when it is hot? Also, is the bottom of the metal pan clean? That is, are we just considering metal on glass friction or is there a layer of organic material between them? $\endgroup$ Jun 2, 2016 at 5:49
  • $\begingroup$ Yes, there is more friction on the hot plate. This occurrs when I move a hot pan off the hot plate to temporarily remove heat and then back on. The pan bottom is made from metal and everything is reasonably clean. $\endgroup$ Jun 2, 2016 at 8:31
  • $\begingroup$ are any of the surface rough? This might be the effect of thermal expansion enlarging the rough bumps on the surface. Just a guess. $\endgroup$ Jun 2, 2016 at 8:59
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    $\begingroup$ I've noticed this with my stovetop also. Cool question. I think the type of heating does not matter. Just the metal to ceramic surfaces that cause this effect. $\endgroup$ Jun 2, 2016 at 13:12
  • $\begingroup$ @Philip_0008 My guess is that your guess is correct. $\endgroup$ Jun 2, 2016 at 15:07

2 Answers 2

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My guess is that it is hot gas or water vaper trapped between the pot / pan and the surface of the range. That layer of gas separates the metal pan from the ceramic / glass range-top and makes it more lubricious.

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  • $\begingroup$ I was going to post a similar answer. I think this is correct, needs some expansion though. I think the grooves probably play a role when the pan is cold, since they can nucleate water condensation. The sliding effect of water films is e.g. used in skiing. $\endgroup$ Aug 9, 2016 at 9:01
  • $\begingroup$ Why couldn't it just work on a macroscopically flat surface. In other words, a surface that cannot be 'rung' together like flat glass? All that may be required is hot gas expanding and a thermal gradient making it expand more than the surrounding gas. Seems like that could easily be the case without macroscopic grooves. $\endgroup$ Aug 10, 2016 at 6:05
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user3533030's answer looks possible, but let me propose an alternative explanation. (I am not claiming this is the right answer!)

Heat may deform the pan, thus reducing the contact surface with the stove and the friction force.

Argument towards this explanation:

Pans are known to change shape with heat (thermal dilation). Some bad pans even make a "clack" noise when dilation makes it necessary for the pan to suddently change its overall shape and take a shape far from a flat surface.

I think you could try some experiments to help determining the cause of the phenomenon:

  • If you heat the stove first, and then put the cold pan on it, is the friction the same as if both where cold?

  • Try to estimate friction between the stove and a clean wiping cloth with both the stove and cloth cold, and then hot. Unless there is some reconfiguration of the fibers of the cloth with heat, I think the contact surface should be the same, so if you see a difference in the friction between cold and hot, this would be in favor of some temperature-dependent behaviour of a thin coating on the stove such as residual oil.

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  • $\begingroup$ Flip over the pan. Then put a straight edge (e.g. metal ruler) across the bottom of the pan. Estimate the sag (run out). Then heat up the pan, and flip it over again and estimate the run out. See if it really does macroscopically curve. $\endgroup$ Oct 6, 2016 at 5:02
  • $\begingroup$ Then again, you can just check to see if it rocks back-and-forth when hot! $\endgroup$ Oct 6, 2016 at 5:03
  • $\begingroup$ @user3533030: I did the rock test: it does not rock. This is a pretty sturdy pan and does not percievably alter its shape when heated. $\endgroup$ Jan 16, 2017 at 20:08

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