Theory tackles how glass remembers earlier forces.
Glass is not as well understood as most materials, because it
straddles the line between liquid and solid. In typical crystalline
materials, molecules assemble into a set structure over the span of the
entire material as the substance solidifies from a disordered liquid
form. Glass, on the other hand, retains a liquid-like disorder even
after it hardens.
Without a set architecture, these disordered molecules are
particularly vulnerable to outside forces. If you push or pull on a
substance, you create internal forces, or stress, in the material
itself. Once you remove that force, you'd expect the molecules to return
to equilibrium, removing the stresses. But glassy materials "remember"
the long-gone force.
Physicists are trying to understand how glass molecules permanently
retain this residual stress. "The material properties depend on how the
material is produced," said Thomas Voigtmann, a physicist at the
University of Konstanz and the Institute of Materials Physics in Space,
in Koln, Germany. "And that's a rather fascinating topic to understand."
In a paper accepted for publication in Physical Review Letters,
Voigtmann and his coauthors describe glass's residual stress in physics
terms, by observing how the motion of individual atoms affects the
entire complex system. But engineers are already taking advantage of
glass's history dependence—no theoretical physics required.
Stress helps glass resist damage. By incorporating it into the
manufacturing process, Engineers at Corning, Inc., in N.Y., can give a
normally fragile material super-strength. Their Gorilla Glass product
now forms the screens of more than 1,000 different devices, from
smartphones to tablets to televisions.
To avoid building flaws into the material, Corning creates large,
flat panes of Gorilla Glass mechanically. During the process, the molten
glass is suspended by its top edge, leaving it untouched by human
hands—or anything else. Despite their stability, these sheets cannot
prevent future damage...yet. The next step is to apply stress to the
glass, compressing its molecules to strengthen the material and enable
it to resist flaws.
Cut to appropriate sizes, the unfinished Gorilla Glass then takes a
bath in a molten solution of potassium salts. This process leaches
small sodium ions out of the glass and replaces them with larger
potassium ions. The large particles squeeze the sheet from the outside
in, compressing the material. This creates two outer layers squeezing
inwards, towards a central layer that balances out the internal forces
by pushing back.
"You have an equilibrium of stress and tension," explained Marcus
Haynes, a senior applications engineer at Corning. "There's a layer of
compressive stress, then a layer of central tension, where the glass
wants to press out, then another layer of compressive stress."
Compressing the surface of the glass makes it stronger, able to resist blows and scratches rather than breaking immediately.
"Even if you damage the glass, the flaw is contained within that
compressive stress layer," Haynes elaborated. "It doesn't allow the flaw
to expand." In order to break a Gorilla Glass screen, a flaw would have
to penetrate through the compressive layer and into the tension layer.
Although the ingredients that go into Gorilla Glass also help the
material withstand damage, stress is the real key to its abilities. It
works even though its creators didn't understand its exact molecular
behavior. But scientists still want to know more.
"We're trying to give some guidance on why this works and how it
can be improved — that's the long term goal," said Voigtmann. "What
we're trying to do is give a theoretical physics explanation for
empirical laws."
To formulate this explanation, the scientists used both theoretical
physics and experimentation. Molecular interactions are particularly
difficult to observe because they occur on such a small scale. Instead
of zooming in to the molecular level, the researchers took advantage of a
glass substitute: colloids. A colloid is a type of substance with
particles suspended in a solution. The common colloid paint, for
example, consists of solid pigments floating in liquid.
"These colloids act like atoms," explained Voigtmann. "It's a model
system that in many respects behaves like window glass, but it's on a
blown-up scale: colloids are big enough to watch under a microscope."
The researchers put colloids under stress and then observed their
behavior through a microscope. This led them to develop a physical
theory that describes why forces in molten glass remain locked in the
material.
Although this theory accurately models the behaviors that the
researchers observed during experiments, the understanding of glass
remains a "hotly debated" topic, said Voigtmann.
- Sophie Bushwick, Inside Science News Service
Sophie Bushwick is a freelance science writer based in New York City. Her work has appeared in numerous print and online outlets.
Alterations to the usual glass production process, such as putting the material under stress, can introduce effects that linger even after the material hardens. While manufacturers have long exploited this phenomenon to strengthen glass. electric glass
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