Contrary to urban legend, the glass in cathedral windows doesn鈥檛 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 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.
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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 of Rutgers University in Piscataway, New Jersey, for atomic rearrangements in glass nearly two decades ago. 鈥淭his is gorgeous work,鈥� says Phillips, who is delighted by the experimental support for his theory.
鈥淓xperimentally it is tour de force,鈥� says 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鈥檚 mathematical theory of atomic rearrangement.
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