Contrary to urban legend, the glass in cathedral windows doesn't flow like fluids. But the of smartphone and tablet screens has been caught moving.
Glass feels solid, but its atoms are arranged randomly, as in a liquid. This dual-personality led to the idea that glass might flow like a fluid, and over hundreds of years, could account for the uneven thicknesses of windows in medieval cathedrals. But this idea was disproved in the 1990s.
Now Doug Allan and colleagues at Corning, the New York-based company that makes super-tough Gorilla Glass, have watched a 1-metre-square sheet of the stuff shrink by 5 micrometres in both width and length, in just 10 days. The process then slowed, taking 18 months to shrink another 5 micrometres.
Allan attributes the movement to sodium and potassium atoms, added to make the glass more durable. When the glass is first made, these atoms are in energetically unfavourable positions. Once they find more favourable spots, interatomic forces trap them, reducing the overall movement, and therefore the size, of a sheet.
Mathematical law
The shrinkage is too small to pose a problem to smartphone owners, says Allan. But it is the first measurement of such movement in glass.
Intriguingly, the rate of movement seems to follow a mathematical power law predicted by James Phillips of Rutgers University in Piscataway, New Jersey, for atomic rearrangements in glass nearly two decades ago. "This is gorgeous work," says Phillips, who is delighted by the experimental support for his theory.
"Experimentally it is tour de force," says Peter Wolynes of Rice University in Houston, Texas, who was not involved in the study. However, Wolynes does not think the data are precise enough to validate Phillips's mathematical theory of atomic rearrangement.
Journal reference: Physical Review Letters, 10.1103/PhysRevLett.110.265901
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