The Glassy State 5/17/21

Science with Richard Bleil

You can probably name three states of matter. Many people believe they are solid (fixed shape and fixed volume), liquid (variable shape but fixed volume as they conform to the shape of the container) and gas (variable shape and variable volume as they expand to fill the shape and volume of their container). But there are more, such as plasma, the state of matter that exists at extreme temperatures where molecules themselves fall apart. You’re familiar with this state, and probably refer to it as “flame”.

Another familiar state is the glassy state. Yes, this refers to glass, but it is probably one of the most unusual states of matter you’ll ever cross. In fact, in graduate school it was my first topic for research. There is, even today, a debate of if the glassy state falls into the category of liquid or solid (it’s neither; it’s the glassy state). Glass seems to be a solid. After all, drinking glasses, vases, even windows are made of glass. They wouldn’t be of much value if they didn’t hold their shape, so of course they’re solid. And yet, graduate students have noted that older glass tubes in the lab tend to be bent, and engineers have noticed that very old windows in very old homes tend to be thicker at the bottom than the top. Of course, the counter argument to each of these observations is that graduate students, when they need a glass tube, are more likely to pick out the best ones which are straight. What’s more, many years ago, manufacturing techniques probably resulted in uneven thickness throughout glass, and the observant builder would realize that the window is more stable with the heavier end down.

My friend, the geologist, countered both of those arguments by saying it depends on the time frame. In geological time frames (millions of years), any object will deform, so there is no solid form. The sturdiest rock will eventually yield, if left untouched, to the forces of gravity flowing downward. Fair enough, so when discussing states of matter, we should only discuss them on a time scale convenient to us as people. In such a time frame, glass is clearly a solid.

But it’s not solid. It’s glass. To explain this a little bit better, we need to understand the process of “quenching” which is how the glassy state is formed. We start with the concept of viscosity, or resistance to flow. We’re familiar with this, too, as we all know that honey flows slower and with more difficulty than water. We say honey has a higher viscosity than water. Materials tend to have a higher viscosity at lower temperatures. This is what quenching is.

We start with, basically, sand, silicon oxide. Sand is opaque; you cannot see through it. But, if we melt it, it forms a liquid, and like so many liquids the molecules in the silicon oxide are randomly distributed throughout the volume of the liquid. In a solid like sand, the molecules align themselves in a regular pattern, like people in a crowded orchestral concert. Shoulder to shoulder, there’s a pattern to where people are sitting as delineated by the chairs in the audience. Molecules tend to do the same thing in crystals; they align themselves in very regular patterns. The shape of a large quartz crystal has the shape it does because that’s how the silicon oxide molecules align themselves at an atomic scale. Salt does the same thing. It’s shaped as tiny little cubes, not because it’s cut that way, but because it grows that was as the tiny molecular cubes grow in an ever-larger cubic shape.

But in a liquid, the molecules are free, like the patrons of the concert in the concert hall’s foyer during intermission. People are randomly distributed, some clumped together with their friends or at the refreshments, some milling about alone. There is no pattern. And unlike being in their seats, they are all free to move as they wish. This is the liquid state, and, as you may have noticed, the liquid state tends to be clear. Not necessarily colorless, but clear. As silicon oxide melts, it forms such a clear liquid. If you’ve ever seen glassblowing, you’ll recognize that this is already a highly viscous liquid, but if it is cooled, it will become even more viscous.

Quenching means to cool very rapidly. If you cool too quickly, the silicon oxide will simply recrystallize, defeating the purpose of the glass. So, it is “quenched”, or cooled very quickly to increase viscosity to such a high level that the molecules themselves are trapped in this liquid arrangement, much like a photo of that foyer during intermission. The people are there, but they’re disordered. In a glass, the molecules are disordered, much like in the liquid state, but cannot move because the viscosity is holding them. This means they cannot rearrange themselves into their crystalline form.

But, they can, albeit excessively slowly. Again, back to the concept of the time frame, the reality is that the silicon oxide can move, just extremely slowly. This is why old glass becomes brittle. Some sub-microscopic regions of the glass will eventually arrange themselves and form crystals so small that they cannot be seen. These highly localized crystalline regions are a different pattern than the liquid-like pattern surrounding them, and as such don’t bond well with their surroundings. This is like a flaw, a region of instability. Over time, enough of these regions result in a brittle and easily broken glass.

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