Steve W. Martin, an associate professor of materials science and engineering
at Iowa State University, was happy to tackle this question:
The seeming paradox that a glass is at the same time a liquid and a solid is
not easily reconciled. Glasses are "solids" produced by cooling a molten
liquid fast enough that crystallization does not occur at the normal
freezing point. Instead, the liquid supercools into the thermodynamic
never-never land of metastability: kinetically settled enough to exist as a
well-defined state of matter, yet not truly thermodynamically stable. As the
supercooled liquid cools to lower and lower temperatures, the viscosity of
the liquid increases dramatically. That happens because as thermal energy
becomes ever less available, chemical bonds within the liquid constrain the
atomic motion more and more.
As the glass cools, the time it needs to demonstrate liquid behavior (the
"viscous relaxation time") increases and eventually reaches extremes. At the
so-called glass transition temperature, the relaxation time is on the order
of a few minutes. On a short timescale, the "liquid" glass will appear
solid, but after a short while, it can be seen to be slowly flowing, like
incredibly thick syrup. At still lower temperatures, the relaxation time
reaches values that are truly geologic, i.e., many millions of years. Window
glass at room temperature has a nearly incalculable relaxation time,
approaching the age of the universe itself. For all practical observations,
this glass is a solid. But its solidity is in the eye of the beholder.
J.-Philipp Crusius
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