Anyone who gets stuck washing/drying the pots and pans after supper may notice that Teflon-coated surfaces seem to dry almost effortlessly. I assume that’s because any water on the surface of the pan ‘partitions’ into the material of the towel preferentially.
But I have also noticed something else - that any little beads of water left on the Teflon-coated surface seem to evaporate extremely quickly (compared to a non-Teflon-coated surface). If I notice some of those little beads, but just leave them alone and come back even after only a very few minutes, they have vanished - I assume, but can’t be certain, that they’ve evaporated.
So, my first question: Does the friction of the evaporating surface influence the rate of evaporation from that surface? More specifically, does water evaporate faster from a Teflon-coated surface than the same surface minus the Teflon?
My second question is a more general one about evaporation: does evaporation cool an object/body primarily because the most energetic (and hottest) water molecules have left the object, so that the mean temperature of those that remain is lower? That is the impression I get from reading the Wikipedia article about the subject, which seems to be at odds with what I remember from high school physics. I was taught that evaporation cools primarily because in order to evaporate, water molecules must take from the body the ‘heat of vaporization’ (enthalpy), a relatively large quantity which even exceeds the energy required to heat the water to the temperature of evaporation in the first place.
I apologize for the length of this post. But, by forcing myself to state explicitly my argument and the assumptions underlying it, I am better able to figure out where my gaps are. And, of course, it also helps anybody who’s trying to teach me (i.e. they have a better sense of where I’m, coming from).
I don’t know about your first question and, truth to tell, about your second one either, but I have a WAG about the latter. Essentially I see no difference between the two explanations. If the more energetic molecules escape then the ones left behind are cooler on average and will cool any surface they are on. That is your second explanation. But in what way are the observed results different from your first? So my WAG is that both are true, just different ways of describing the same phenomenon.
For the first question, I believe it is the fact that the water is beading and increasing its exposed surface area that is making the difference. The greater the exposed surface area, the greater the rate of evaporation.
For the second question, I was taught the same as you in school.
How sure are you that your first question even accurately describes a phenomenon? Have you compared small amounts of water on teflon and non-teflon pans at the same temperature and observed the disappearing at different rates? From your description your observations seem to me to be nowhere near rigorous enough to even suspect a phenomenon, but supposing there is, I’d WAG it being due to the beads on teflon actually beading, leaving a larger surface exposed to air. (A flattened “bead” on non-non-stick would have a larger surface, but only some of it would be exposed to air.)
As for your second question, how are you imagining the water takes the heat from the body?
Re Q#1: Maybe the water is just rolling off more easily? On a regular material, the water will adhere to the pot, but not so much on a Teflon one. Here’s how to test it: Lay the utensil horizontally, and have a similar amount of wetness on both the top and bottom. The only thing drying the top will be evaporation, but the bottom will have both evaporation and gravity.
Beading would seem to me to decrease the exposed surface area. If the water doesn’t bead, it would be spread very thinly over a much larger surface area.
As others have stated, these are just two different ways of describing the same process. Though in the first description, one should also add that the molecules left behind re-distribute themselves back into a thermal distribution, so there are again molecules above the threshold needed to escape, and the process continues.
Yes, a sphere is the smallest surface area for a given volume.
I think the main issue is that less surface adhesion means more water runs off, and the remaining water beads more which makes it visible. But there’s less water, so it dries faster.
But I am curious if adhesion affects evaporation. I have a plastic insulated travel cup. After using and rinsing it, it seems drops will stay in the bottom of that cup for a long time before drying - hours. Generally right in the crevise at the bottom. The region with the most cup surface to bond to. Water will generally stay in cracks longer than on an exposed surface.
For a free-floating volume. For a bead of water, the portion of the surface adhering to the surface doesn’t contribute to evaporation, so a hemisphere would have the smallest exposed surface area. Drops in a corner or a crack can have an even lower area for a given volume.
I suspect the reason water evaporates more slowly from drops on plastic is that plastic is not a good thermal conductor. A little water evaporates, cooling the bead, and it stays cooler. For a bead on metal, or a thin layer of teflon over metal, the pan acts as a large heat sink (well, cold sink), warming the bead back up.
I have observed Q1 as well - I think it’s true that teflon pans dry faster.
My explanation has always been the assumption that there was less water there to begin with. If a cast iron pan holds onto 2 g of water and the teflon pan only holds 1 g of water, then we expect the teflon pan to be finished evaporating faster simply because it doesn’t have as much evaporation to do.
I really don’t think any other explanation is needed, though I suppose the shape of a drop could have a small effect.
Ah, but the surface has that tiny patina that lets you CREATE these tiny beads of water. and the rest of the surface is dry. If you try it on another material , do you get the same amount of water ?
Its not just in the the size of the droplets that you can see, although that is the first thing to be sure is the same and quite difficult to be sure of…
Say you tried to do the same thing with wood. The wood may have the similar patina but the wood itself is absorbing water and so the entire surface is wet… there is more water there to dry…