Ice isn’t always ice all the way. Even at temperatures well below freezing, its surface can be coated with a film of quasi-liquid atoms, typically only a few nanometers thick.
The process of its formation is known as pre-melting (or “surface melting”) and this is why your ice cubes can stick together even in the freezer.
In addition to ice, we have observed a premolten surface layer in a wide range of materials with crystalline structures, those where the atoms inside are arranged in a neat lattice, such as diamonds, quartz, and table salt.
Now, for the first time, scientists have observed surface melting in a substance found in internal fragments: glass.
Glass and ice may look very similar, but they are often very different at the atomic scale. Where crystalline ice is nice and neat, glass is what we call an amorphous solid: it has no real atomic structure to speak of. Instead, its atoms are simply squished together, more like you’d expect to see in a liquid.
This, as one might expect, makes it much more difficult to detect a quasi-liquid premelt film on the glass surface.
Detection of this liquid membrane layer is usually done by experiments involving neutron or X-ray scattering, which are sensitive to atomic order.
Solid ice is ordered. surface melting is less. In glass, everything is crap, so dispersion wouldn’t be a particularly useful tool.
Physicists Clemens Bechinger and Li Tian of the University of Konstanz in Germany took a different approach. Instead of investigating a piece of atomic glass, they created something called colloidal glass—a suspension of tiny glass spheres suspended in a liquid that behaves like the atoms in atomic glass.
Since spheres are 10,000 times larger than atoms, their behavior can be seen directly under the microscope and therefore studied in greater detail.
Using microscopy and scattering, Bechinger and Tian closely examined their colloidal glass and detected signs of surface melting. That is, the particles on the surface moved faster than the particles in the bulk glass below it.
This was not unexpected. The bulk glass density is greater than the surface density, meaning that the surface particles literally have more room to move. However, in a layer below the surface, up to 30 particle diameters thick, the particles continue to move faster than the bulk glass, even when they reach glass bulk density.
“Our results show that surface melting of glasses is qualitatively different compared to crystals and leads to the formation of a surface glassy layer,” the researchers write in their paper.
“This layer contains cooperative clusters of highly mobile particles that form at the surface and propagate deep into the material by several tens of particle diameters and well beyond the region where the particle density saturates.”
Since surface melting changes the surface properties of a material, the results offer a better understanding of glass, which is extremely useful in a number of applications but also quite strange.
For example, high surface mobility could explain why thin polymeric and metallic glassy films have high ionic conductivity compared to thick films. We already use this property in batteries, where these membranes act as ion conductors.
A deeper understanding of this property, what causes it and how it can be caused will help scientists find optimized and even new ways to use it.
The team’s research was published in Nature communications.
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