Strange phenomenon of liquid skin discovered on glass surface : ScienceAlert

Ice is not always ice everywhere. Even at temperatures well below freezing, the surface can be covered with a film of quasi-liquid atoms, the thickness of which is usually only a few nanometers.

The process of its formation is known as pre-melting (or “surface melting”), 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 variety of materials with crystalline structures, those where the atoms inside are arranged in a neatly ordered lattice, such as diamonds, quartz and table salt.

Now, for the first time, scientists have observed surface melting in a substance that’s in internal shambles: glass.

Glass and ice can be very similar, but on an atomic scale they are often very different. Where crystalline ice is nice and neat, glass is what we call a amorphous solid: It has no real atomic structure to speak of. Instead, the atoms are a bit all skewed, more like you’d expect in a liquid.

This, as you might expect, makes it much more difficult to spot a quasi-liquid pre-melted film on the glass surface.

The detection of this film-like liquid layer is usually done through experiments with scattering neutrons or X-rays, which are sensitive to atomic order.

Solid ice is ordered; the melting of the surface is less. In glass, it’s all a mess, so scattering 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 examining a piece of atomic glass, they created something called colloidal glass — a suspension of microscopic glass spheres suspended in a liquid that behaves like the atoms in atomic glass.

Because the 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 carefully examined their colloidal glass and identified the signs of surface melting; namely, the particles on the surface moved faster than the particles in the bulk glass below.

This was not unexpected. The density of the bulk glass is higher than the density of the surface, literally giving the surface particles 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 densities.

“Our results show that the melting of glass at the surface is qualitatively different compared to crystals and leads to the formation of a glassy surface layer,” the researchers write in their paper.

“This layer contains cooperative clusters of highly mobile particles that form at the surface and spread deep into the material by several tens of particle diameters and well beyond the region where the particle density saturates.”

Because surface melting changes the surface properties of a material, the results provide a better understanding of glass, which is extremely useful for a range of applications, but also pretty wacky.

For example, high surface mobility could explain why thin polymeric and metallic glassy films have high ionic conductivity compared to thick films. We are already applying this property in batteries, where these films act as ion conductors.

A deeper understanding of this property, what causes it and how it might be caused will help scientists find optimized and even new ways to use it.

The team’s research was published in nature communication.