Ice is not always ice. Even at temperatures well below freezing, its surface can be covered with a film of semi-liquid atoms, usually only a few nanometers thick. The formation process is known as pre-melting (or “surface melting”), and this is why your ice cubes can stick together, even in the freezer. Besides ice, we have observed a pre-molten surface layer in a wide variety of crystalline materials in which the atoms are arranged in a neatly arranged lattice, such as diamond, quartz, and table salt.

Now, scientists have observed for the first time the surface melting of a substance in inner chaos: glass.

Glass and ice may look very similar, but they are often very different on the atomic scale. Where crystalline ice is beautiful and smooth, glass is what we call amorphous solid: It has no real atomic structure. Instead, its atoms are simply compressed together, similar to what you would expect to see in a liquid. This, as expected, makes it much more difficult to detect the semi-liquid premolten film on the glass surface. Detection of this liquid film is usually done using atomic arrangement sensitive neutron or X-ray scattering experiments. Hard ice is ordered; surface melting is less. Glass is messy, so scattering wouldn’t be a particularly useful tool.

Physicists Clemens Boehinger and Li Tian of the University of Konstanz in Germany took a different approach. Instead of probing a piece of atomic glass, they created something called colloidal glass—a suspension of microscopic glass spheres suspended in a liquid that behaves like atoms in atomic glass. Because 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, Behinger and Tian examined their colloidal glass and found signs of melting on the surface; that is, particles on the surface moved faster than particles in the mass of glass underneath. This was not unexpected. The density of cast glass is higher than the surface density, which means that the surface particles literally have more room to move around. However, in a subsurface layer with a thickness of up to 30 particle diameters, the particles continue to move faster than in cast glass even when they reach the bulk density of the glass.

“Our results show that the surface melting of glass is qualitatively different from that of crystals, leading to the formation of a surface glass layer,” the researchers write in their paper.

“This layer contains collaborative clusters of highly mobile particles that form at the surface and diffuse deep into the material for tens of particle diameters and far beyond the region where the particle density is saturated.”

As surface melting changes the surface properties of the material, the results provide a better understanding of the glass; this is extremely useful in many applications, but quite surprising. For example, the high surface mobility may explain why thin polymer and metallic glass films exhibit high ionic conductivity compared to thick films. We already use this property in batteries where these films act as ion conductors. A deeper understanding of this feature, what causes it and how it can be triggered, will help scientists find optimized and even new ways to use it.