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I was cooking my breakfast this morning. As usual I put some coconut oil on the pan and let it melt. I moved the oil around to cover the whole pan. Once placed back on the stove the oil contracted into one unified "puddle" of coconut oil in the centre of the pan. I repeated and the same thing happened?

Why is this? Is it to do with the change in temperature once the pan is lifted off the stove?

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    $\begingroup$ There are a few mechanisms that could explain it. Firstly, what setting is the stove at? $\endgroup$ – TotallyRhombus Apr 27 '17 at 22:54
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    $\begingroup$ Probably gravity. My oils and butters pool towards the back of the pan $\endgroup$ – Kyle Kanos Apr 28 '17 at 0:45
  • $\begingroup$ Was this a non-stick pan? $\endgroup$ – John Rennie Apr 28 '17 at 10:52
  • $\begingroup$ @JohnRennie I don't know how this comment got by me - it was indeed a non-stick pan. $\endgroup$ – Rumplestillskin Jul 10 '17 at 9:54
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Cooking oil on a PTFE surface has a high contact angle. That means if you spread out a thin film of oil it will contract into a more compact form. This is what you are seeing in our (non-stick) pan. This shows what is happening:

Oil film

Because the PTFE-oil contact angle is greater than 90º the force due to the oil/air surface tension (the red arrow) points inwards towards the centre of the film. This force will pull the edge of the oil film inwards so it shrinks into a smaller thicker puddle.

The fact it happened when you put the pan on the heat is because the increased temperature reduces the viscosity of the oil so it flows more rapidly. If you left the cold pan for a few hours you would also see the oil on it shrink back into smaller puddles.

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  • $\begingroup$ +1 Excellent answer! It seems a lot of my questions involve the kitchen! $\endgroup$ – Rumplestillskin Jul 10 '17 at 10:07

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