Ice isn’t always ice all the way. Even at temperatures well below the freezing point, its surface can be covered with a film of quasi-liquid atoms, its thickness being generally 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 premelted surface layer in a wide range of materials with crystalline structures, those where the atoms within are arranged in a carefully ordered lattice, such as diamonds, quartz and rock salt. table.
Now, for the first time, scientists have observed the surface melting of a substance that is internally disordered: glass.
Glass and ice may look very similar, but they are often very different on the 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. Instead, its atoms are all kind of huddled together, more like you’d expect to see in a liquid.
This, as might be expected, makes it much more difficult to spot a quasi-liquid pre-melted film on the surface of the glass.
Detection of this liquid film layer is usually performed by experiments involving neutron scattering or X-rays, which are sensitive to atomic order.
Solid ice is ordered; superficial fusion is less so. Everything in glass is a mess, so diffusion wouldn’t be a particularly useful tool.
Physicists Clemens Bechinger and Li Tian from the University of Konstanz in Germany took a different approach. Rather than 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 the atoms in atomic glass.
As 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 identified signs of surface melting; that is, the particles on the surface moved faster than the particles in the bulk glass below.
It was not unexpected. The bulk glass density is greater than the surface density, which means surface particles literally have more room to move around. However, in a layer below the surface, up to 30 particle diameters thick, particles continue to move faster than bulk glass, even when they reach bulk glass densities.
“Our results demonstrate that the surface melting of glasses is qualitatively different from that of crystals and leads to the formation of a vitreous layer on the surface”, write the researchers in their article.
“This layer contains cooperative clusters of highly mobile particles that form at the surface and proliferate deep into the material over 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 provide a better understanding of glass, which is extremely useful in a range of applications but also quite wacky.
For example, high surface mobility could explain why glassy polymeric and thin metallic films have high ionic conductivity compared to thick films. We are already taking advantage of this property in batteries, where these films act as ion conductors.
A better understanding of this property, its causes and how it can be induced will help scientists find optimized and even new ways to use it.
The team’s research has been published in Nature Communication.