(ORDO NEWS) — Ice doesn’t always stay ice. Even at temperatures well below freezing, its surface can be covered with a film of quasi-liquid atoms, typically only a few nanometers thick.
The process of its formation is known as premelting (or “surface melting”). melts”), which is why your ice cubes can stick together even in the freezer.
In addition to ice, we have observed a pre-melted surface layer in a wide range of materials with a crystalline structure, such as the atoms inside are arranged in a neatly ordered lattice, like diamonds, quartz and table salt.
Now, for the first time, scientists have observed the surface melting of a substance that is in internal turmoil: glass.
Glass and ice may look very similar, but they are often very different on an atomic scale. Where crystalline ice is nice and tidy, glass is what we call an amorphous solid: it has no real atomic structure to speak of.
On the contrary, its atoms are just sort of jumbled together, more like what you would expect to see in a liquid.
This, as one would expect, makes it much more difficult to detect the quasi-pre-melted liquid film on the glass surface.
Detection of this filmy liquid layer is usually done by experiments involving neutron or X-ray scattering, which are sensitive to atomic order.
Hard ice ordered; surface melting is less. The glass is a mess, so scattering won’t be a particularly useful tool.
Physicists Clemens Bechinger and Li Tian from the University of Konstanz in Germany took a different approach.
Instead of examining a piece of atomic glass, they created what is called colloidal glass, a suspension of microscopic glass spheres suspended in a liquid that behaves like atoms in atomic glass.
Since spheres are 10,000 times larger than atoms, their behavior can be seen directly under a microscope and therefore studied in more detail.
Using microscopy and scattering, Bechinger and Tian closely examined their colloidal glass, and they found signs of superficial melting; namely, the particles on the surface moved faster than the particles in the volume of glass underneath.
It wasn’t a surprise. The density of bulk glass is higher than that of the surface, which means that 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 bulk glass density.
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