Qfyilyi

According to many high school textbook sources, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface. Students are often made to memorize this fact, and many physics questions use it as an assumption.

However, it's perfectly obvious that this doesn't actually happen. If you put a drop of water on glass, it might spread out a little, but it doesn't remotely coat the whole thing. In fact, I've never seen anything like the phenomenon described as "perfect wetting".

Can perfect wetting actually be observed for everyday materials? If not, what are the main additional factors preventing it from happening, as the textbooks say?

In everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounter a clean glass surface.

I spent many years carrying out experiments involving interactions with glass surfaces, and to get the surface clean we had to clean it with chromic acid. A quick Google found instructions for doing this here, but if you ever feel tempted to try this at home do note the comment in that article:

The dichromate should be handled with extreme care because it is a powerful corrosive and carcinogen.

If you survive the cleaning process then you will find a water drop placed on the glass does have an effectively zero contact angle and the drop will spread out almost completely.

But it's only under these extreme conditions that you will see this. Just leaving the glass exposed to the air for a few hours is enough to coat it with a monolayer of whatever organic detritus if floating around (which if humans are present is quite a lot :-). Once this happens you aren't measuring the contact angle on glass, you are measuring it on whatever organic film is coating the glass.

It's because of surface energy

You can just cover glass with a thin layer of material that has low surface energy (like teflon) and then water/stuff won't stick(will stick alot less) to it...

Check this link for much more information.

To answer the question, yes and no.

The reason why we don't observe water, spread out on its own, on glass, as much as it can, is because of >> see accepted answer :D

However if you spread it yourself, then yea you could make a very thin coat of water covering the whole glass, which could be kinda described as perfect wetting.

I'm gonna say it's due to surface tension. In darkroom photography unless you're careful you'll get waterdrop marks on your negatives when you hang them up to dry. There's a commercial product called Photoflo (sp?) that stops this. Being cheap I found a couple drops of ordinary liquid dish detergent in the last rinse does about the same thing a lot cheaper. There are also "wetting agents" or some similar name for products you put into your dishwasher to avoid water spots on glasses, same idea. This is also related to a classic experiment to measure the thickness of an oleic acid molecule by putting a drop into a water bath and measuring the area it spreads out to. 8th grade science, 50 years ago but I still remember it.