Why doesn't the windshield fog up where my kid drew on it with her fingers?

Question

A few weeks ago the inside of my car windshield was fogged up and my older kid used her finger to draw a face in the condensation.

Weeks later, the windshield fogged up again, and the face became visible again. My younger kid asked why.

“The older kid's fingers left an oily residue on the windshield,” I said, which seemed to satisfy her.

But it doesn't satisfy me. Does the oily residue prevent further condensation? If so, why? The oil droplets are just as cold as the glass of the windshield.

And if the water vapor in the air can't condense where there's oil, because of some electrostatic property of the oil molecules, why doesn't it condense in the space between the microscopic oil droplets? If it did, I would expect that part of the glass to be more foggy because there would be a nonuniform film of oil and water droplets of different sizes.

What's going on here?

Answer 1

An oily surface is poorly-wetted by water molecules whereas a glass surface is well-wetted by water molecules. This is because glass consists of silicon, calcium, sodium and aluminum oxides. This furnishes a surface with a lot of oxygens sticking their butts out into the bigger world beyond. Those oxygens look kind of like water molecules to other water molecules in the vicinity, which then will try to preferentially populate that surface.

An oil-covered surface presents almost nothing but hydrogens to the world which do not have that (weak) bonding tendency, and so water molecules pretty much "ignore" such a surface.

In fact, a glass surface will spontaneously load itself with moisture drawn from the air. If this happens while sandwich ("safety") glass is being pressed together, the water monolayer will prevent the inner plastic core from bonding to the outer glass panels and the sheet will delaminate.

So glass surfaces are usually loaded with water molecules and, hence, they "look" just like more water to another water molecule, meaning that water smeared on glass will form a film on the glass that sticks to it.

Now if you smear something oily on top of that, the surface no longer looks like water which means water molecules will not readily condense on it. This is because they have no affinity for the oil, as they do for the water-loaded glass.

This means that an oily smear on glass will persistently inhibit condensation of water vapor to liquid water on it until you clean it off. My son's feet prints on the inside of our minivan windshield would stay condensate-free this way for 6 months or more.

Answer 2

The hydrophobic effect is the technical name for the reason why oil and water don't mix. It's probably also the reason why reason why water droplets do not condense on oily surfaces. The most common explanation is that the effect is due to the entropy of the water molecules. To explain more simply, consider a water molecule that is either (1) next to an oily surface or (2) surrounded by other water molecules:

1. A water molecule next to an oily surface will orient itself relative to that surface. This is due to the fact that water is a polar molecule (i.e. it has one end that more positively charged and another that's more negatively charged) and so one end will have more affinity for the oil than the other.

2. A water molecule surrounded by other water molecules will not favor any particular orientation.

This means that a water molecule has more "freedom of movement", i.e. more entropy, in solution than at an oil-water interface. As a result, the lowest energy state will be the one that minimizes the surface area of any oil-water interfaces*. In this case, that means having water condense on the glass instead of on the fingerprints.

You also ask why water droplets don't condense between the oily droplets. I don't think there are oily droplets. Oils are well-known to form membranes, and so the entire surface touched by a finger is probably coated in oil. The reason why oils form membranes turns out to also be the hydrophobic effect, combined with the fact that an "oil" is, more precisely, a molecule where one end can mix with water and the other can't.

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*I'm kind-of glossing over why more entropy means lower energy. Loosely, you can think of every allowed molecular configuration being equally likely, and so macroscopic states with more underlying molecular configurations end up being more likely overall. But this is getting outside of what I feel qualified to talk about.