Why won’t two pieces of glass slide against one another?

SDStaff DavidB replies:

Before I tackle your question, I want to make something perfectly clear to everybody reading this. Glass is not a liquid! It is an amorphous solid. Deal with it, folks.

Now, with that off my chest, you might think this would be an easy question to answer, especially for a guy who got his degree in ceramic engineering (yes, really), took several classes about glass, and even did research work in the varying effects of additives to phosphoborosilicate glasses. But one property I never studied was glass-on-glass friction and precisely none of my glass textbooks contain anything on the subject. While I haven’t done a full study of the literature, it seems to be an area just begging for further study, Ian, so if you’re not in college yet, I encourage you to consider going into that field. Fame and fortune (or at least a tenure-track position) await.

Meanwhile, I think I have a likely answer. Basically, without going into too much chemistry, it looks like we’re dealing with a property due to the particular nature of the glass surface. The surface structure tends to have “dangling” bonds. In your standard silicate glass (the glass you are probably most familiar with), there are Si-O- and Si- bonds that are kind of left hanging there (that would be Silicon-Oxygen- and Silicon- for those of you not familiar with the periodic table). When the glass is exposed to water in the air, these leftover bonds react with it to form satisfied SiOH groups. This means the surface of your standard piece of glass is composed of a “hydrated surface layer” (ref: Glass Science, by Robert H. Doremus).

This surface layer is not noticeable in most glasses, but the glasses I worked with were positively sticky in some cases and I was trying to keep them dry! Also, if I recall correctly, some glass manufacturers actually manipulate this layer to get certain surface properties. For example, a windshield manufacturer might manipulate it to help rainwater roll off the window more easily.

So looking at glass this way, you can understand how two pieces of glass both with their sticky hydrated surface layers would tend to rub against each other to create friction. In other words, glass may feel smooth as ice to you, but on a molecular scale there is a definite cause for friction between two pieces.

SDStaff DavidB, Straight Dope Science Advisory Board

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