[jodan63]

Why do some windows bead and some sheet?

[michael papworth]

There are two different classes of glass: hydrophilic and hydrophobic.

Hydro means water. Philic means to like or to attract. Phobic means to dislike or to avoid.

* Hydrophilic glass "likes" water, and water will wet the surface: water will sheet down the glass.

* Hydrophobic glass "dislikes" water and the water will not wet the surface: water will form beads.

It is actually a little more complex than this as it depends on many different factors.

If you're interested, I'll explain it in a little more detail later.

[Tom Mac]

I asked my 4 year old grandaughter that same question. She replied 'because'.

[cat9921]

Why do some windows bead and some sheet?

I was going to say the same as Wally the Window Washer 
But now I am not too sure you see I have a few houses where the water beads and sheets  : on the same glass  one side sheets and the other beads.. So it can not be 2 different types of glass on this window

But whether the water beads or sheets they both come up well with WFP 

[michael papworth]

Why do some windows bead and some sheet?

I was going to say the same as Wally the Window Washer 
But now I am not too sure you see I have a few houses where the water beads and sheets  : on the same glass  one side sheets and the other beads.. So it can not be 2 different types of glass on this window

But whether the water beads or sheets they both come up well with WFP 

Yes, you are absolutley right. I've noticed that in many DG units some sheets of glass are hydrophobic on the outer edges and hydrohilic in the centre. - and I'm puzzled as to why this should be. I can only guess that it's caused in the manufacuring process somehow.

I wonder if silicone being deposited on the outside edges has anything to do with it?

[michael papworth]

OK. Here's the science behind it. There are two things to understand straight away.

First of all, it's important to know that it's a SURFACE effect. Even a few molecules of coating or impurity on the glass surface will have a dramatic effect on what you see.

Secondly, the purity of the water and the cleanliness of the glass surface are both factors. That's why you will notice that the water behaves differently between the moment you start cleaning the window (when it is dirty) and when it's finished and clean.

Do you remember seeing an experiment at school when the teacher floated a needle on water? That's possible due to a phenomenon called surface tension. This is due to the water molecules being attracted to each other. This causes the water to form a "skin" at the surface. It's important in lots of different applications.

For an excellent discussion of surface tension see: en.wikipedia.org/wiki/Surface_tension. If you ignore the mathematical bits, the description is really excellent. I especially recommend the bit on contact angles.

What happens on the glass surface is that the molecules of the water are attracted to each other - hence surface tension, and they are also attracted to the molecules on the surface of the glass. In some complex way that I really don't understand very well, these two different attractions "fight" with each other. This causes the droplets of water to make contact with the glass surface at some angle that depends on the strength of the attraction between the water and the glass.



If the attraction of the water molecules to the glass molecules is weak, the contact angle is high (greater than 90 degrees) and the glass is hydrophobic.

If the attraction of the water molecules to the glass molecules is strong, the contact angle is low (less than 90 degrees) and the glass is hydrophilic.

Phew!!!

Interesting things:

1 - Self-cleaning glass is made by depositing a very thin layer of titanium dioxide (TiO₂) on the surface of the glass. This stuff is highly hydrophilic, so the contact angle betwen the water and the TiO₂ is so small, that the water just slides down the vertical surface.

2 - Putting detergent into the water reduces its surface tension dramatically. That's why soapy water will wet hydrophobic glass. (It's a little more complex than that, but it's near enough.)

[[GQC] Tim]

Very interesting Wally, thanks for that.

Makes me wonder what exactly happens during the manufacturing process that makes it bead or sheet.

[michael papworth]

Yes, I've been looking for info on just this point - and can't find any.

This is only a guess. I think that it's a combination of two things: chemical composition and surface finish.

Chemical composition is straightforward enough to understand. Different chemicals (and therefore molecular shape) will have different attractive forces to the water molecule.

The surface finish is another matter ... and one that I don't really understand. It has somethimng to do with the fact that the surface of glass is optically smooth - to the order of microns (0.000 001 m) However, on the molecular scale, (nanometres - 0.000 000 001 m) it is far from smooth.

Glass can be given surface treatments to make it hydrophobic or hydrophilic. That is another huge topic in itself.

I was chatting to someone today about just this point. He said - "Oh, so hydrophobic glass repels water, does it?" He was thinking of his water repellant raincoat.

And no, it doesn't. There is no such thing as being water repellant. It's just that it isn't strongly attractive to water. The water molecules are more strongly attracted to each other than to the glass molecules. That's why they form beads. They are spherical (ish) because the sphere is the shape which has minimum surface energy for a given volume. Don't ask why!!!!! The answer will be .. "Because!!"

[Jeff Brimble]

Hmm... once the glass has left the float manufacurers in pristine phillic state.... the installers/finishers/housewives  get ther hands on it and xxx it up completely for ever by polishing it or by applying some clever coating on it.

[RO-Sheen]

Well explained Wally. Interesting read!

[cat9921]

Yes, I've been looking for info on just this point - and can't find any.

This is only a guess. I think that it's a combination of two things: chemical composition and surface finish.

Chemical composition is straightforward enough to understand. Different chemicals (and therefore molecular shape) will have different attractive forces to the water molecule.

The surface finish is another matter ... and one that I don't really understand. It has somethimng to do with the fact that the surface of glass is optically smooth - to the order of microns (0.000 001 m) However, on the molecular scale, (nanometres - 0.000 000 001 m) it is far from smooth.

Glass can be given surface treatments to make it hydrophobic or hydrophilic. That is another huge topic in itself.

I was chatting to someone today about just this point. He said - "Oh, so hydrophobic glass repels water, does it?" He was thinking of his water repellant raincoat.

And no, it doesn't. There is no such thing as being water repellant. It's just that it isn't strongly attractive to water. The water molecules are more strongly attracted to each other than to the glass molecules. That's why they form beads. They are spherical (ish) because the sphere is the shape which has minimum surface energy for a given volume. Don't ask why!!!!! The answer will be .. "Because!!"

I can not believe you have the name Wally when you are that intelegant 

I needed to get the dictionary out to understand half of what you said.... Great read buy the way Thanks 

[Neil271052]

Thanks teacher Wally.

Very interesting and intelligent post. 

[Dave Turley]

thanks wally!

now I need to lie down for a while, my head hurts!   lol