A classy material | Nature Physics

Glass, now celebrated with a dedicated International Year, continues to fascinate.

Glass, as we know it today in the form of tableware, windows and optical components, goes back to at least the Late Bronze Age. The archetypical form of glass is silicate (silicon dioxide). Adding other substances alters its processing and properties. Nowadays, glass refers to a larger category of materials, including non-silicate inorganic, metallic and polymeric glasses — the unifying property is the lack of long-range order.

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Rightly so, glass is getting extra attention in 2022, which the United Nations has declared the International Year of Glass (IYoG) (https://www.iyog2022.org/). The initiative builds on the thissis put forward in 2016 that society is now truly in a ‘Glass Age’1, as can for example be appreciated from the impact glass optical-fibre technology has had on global communications and the Internet. Moreover, glass is poised to play a key role in global sustainability challenges, including better healthcare, cleaner energy and more efficient water management, to name but a few.

As befits an International Year, plenty of activities have been planned. Interestingly, most of them focus on the role of glass in art and design. This shouldn’t come as a surprise, as art and technology have always gone hand in hand. For artists, designers and architects, glass has always been a prime material to work with. (Pictured is a fragment of Peace, a stained-glass window by Marc Chagall given to the United Nations in 1964.) One of the major appointments on the IYoG calendar focussing on glass science is the 26th International Congress on Glass, taking place in Berlin next month (https:// www.hvg-dgg-events.com/icg2022). A glance at the meeting’s program reveals how broad glass science is, and how much of it is focused on industrial aspects like sustainable glass production, quality control and recycling.

But glass is not only of industrial interest, it also attracts scientists for the fundamental physics it harbors. The very glass transition itself — the formation of a glass from a liquid upon cooling — remains to be fully understood. Philip W. Anderson’s 1995 statement that “the deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition” still stands2.

A prominent academic problem related to the physics of glasses is the so-called boson peak, an excess in the vibrational density of states at low frequencies of an amorphous solid like glass. In a crystal, at low temperatures, the vibrational density of states increases quadratically with frequency, a dependence that follows from Peter Debye’s ‘phonons in a box’ model for the specific heat of a solid. The origin of the non-phononic contributions, that is, the boson peak, has been the topic of much debate over the years, resulting in a steady stream of papers. Incidentally, in an article published in this issue of Nature PhysicsYuan-Chao Hu and Hajime Tanaka corroborate the plausibility of an earlier explanation of the boson peak, namely that localized excitations of small groups of particles (‘stringlets’) can cause the excess in vibrational density3.

The paper by Hu and Tanaka is purely numerical — glassy systems are well suited for simulation studies, as it’s relatively easy to let a large set of particles interact and see what happens over time, provided there’s enough computer power. This resource constraint is of course a key issue. As Lothar Wondraczek points out in the accompanying News & Views, Hu and Tanaka’s simulated system of up to 33,000 measuring atoms corresponds to a square, single-layer of glass approximately 20 nm by 20 nm, which is a far cry from bulk glass4.

The type of real-world glass that is arguably ideally suited for combined experimental and computational studies are colloidal glasses. These are in essence suspensions of colloidal particles, with typical diameters ranging from nano- to micrometres. Glassy states form by centrifuging the mixture, and can be observed by means of confocal microscopy, enabling to monitor the evolution of the system over hours or even days. Conveniently, the interaction potential between the colloids can be tailored by chemical treatment, so both hard-sphere and long-range interaction situations can be studied experimentally, and simulated highly realistically, providing insights also relevant to the ever-growing wider category of glassy materials .

Our understanding of glasses has come a long way since William H. Zachariasen deciphered the structure of silicate glass 90 years ago5. The focus on disorder has shifted to that of order in disorder and even the notion of a ‘glassy genome’6and the desire for a more inclusive generalized crystallography also related to non-crystalline materials has emerged7. All in all, it’s fair to say that glass has continually been driving science forward. And that indeed, anno 2022, glass gives us plenty of reasons to celebrate.


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A classy material.
Nat. Phys. 18, 603 (2022). https://doi.org/10.1038/s41567-022-01654-4

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