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Glass physicists baffled – the first discovery of surface melting in glass brings unexpected results

Ribbons of fast particles: Physicists have shown for the first time that even amorphous glass exhibits surface melting below freezing. A thin layer of highly mobile liquid particles develops on the surface of the solid. What is surprising, however, is that these fast-moving particles spread out to the depths and form a striped layer with solid and liquid properties. This is a new phenomenon so far.

It has been known since 1842 that the surface of water ice is always flat, even at temperatures below freezing. thin layer of water we take. It allows us to glide on the ice and stick snowballs together. In the meantime, the physicists explained why this surface melted and also showed it in other crystalline solids. According to this, the molecules in the boundary layer are in constant transition states of sublimation and freezing.

Colloidal glass as a test case

However, it was not previously clear whether this surface melting also occurs glasses They are substances that, unlike crystals, do not form an ordered lattice when they solidify, but remain amorphous. Unlike crystals, the structure of the liquid boundary layer does not differ from the rest of the solid glass in its X-ray scattering. According to the theory, such amorphous bottles should also show surface fusion, but this has yet to be proven.

But now Li Tian and Clemens Bechinger of Konstanz University have succeeded. Because real glass atoms are too small to study their behavior directly, they used colloidal glass in their experiment. This material consists of 2 to 3 micrometers of tiny glass beads in a special solvent and behaves like regular glass. On the contrary, colloidal glass particles can be observed under the microscope.

In the experiment, colloidal glass was first cooled to minus 262 degrees and then slowly heated to 259 degrees in 0.2-degree increments – a temperature still below the freezing point of this colloidal glass.

Under the surface surprise

It turns out: “Similar to crystals, we observe glass surface melting, that is, the formation of a liquid layer on the surface below the melting temperature,” according to Tian and Bichinger. The particles in this liquid layer move much faster than the solid below. This proves that amorphous glass also forms such a layer.

Surprisingly, the fast particles (red) are scattered far into the glass and form a region with both liquid and solid properties. How thick this area is depends on the temperature. © Tian and Bechinger/Nature Communications, CC by 4.0

What is surprising, however, is that beneath this thin liquid surface layer, the colloidal glass has taken on a previously unknown state. “Above the solid, we found an unexpected region with the same density as the solid particles, but the particle dynamics are much faster,” the researchers report. The newly discovered layer thus combines liquid and solid properties and continues as streaks of up to 30 particles in diameter farther into the deeper regions of the solid.

fast moving particles

More detailed analyzes revealed that this new layer forms because fast gas-like particles migrate from the surface to the depths. “These 10 percent of the fastest particles form connected clusters and exhibit cooperative dynamics,” Tian and Bichinger report. They explain that the thickness of this unexpected layer varies non-unitally with the effective temperature and is greater near the transition temperature of the glass.

“Because the thickness of this layer can be up to tens of particle diameters, this layer could explain some hitherto poorly understood properties of thin glass films used in many technical applications,” say the physicists. Thin glasses are used in batteries, among other things, as they have a much higher ionic conductivity than thicker layers. With the knowledge now gained, this behavior can now be understood quantitatively for the first time. The newly discovered behavior could also play an important role for stimuli, sensors, or electrodes. (Nature Communications, 2022; doi: 10.1038/s41467-022-34317-2)

Source: University of Constance