Glass and crystals, order changes everything

Dr. Fernando Donado Pérez
PhD in Science (Physics) from the Benemérita Universidad Autónoma de Puebla (BUAP).

A diamond, an emerald, a ruby are precious stones that enrapture, that attract, that conquer, objects that we wish to have and admire. Objects that we wish to have and admire. Where does their beauty lie? Why the blood shed to possess them?

The diamond is nothing more than a collection of carbon atoms equal to those contained in a pencil lead, equal to those contained in a piece of coal. So what is the difference between these materials? The order! That is the difference! The order changes everything. In diamond the atoms are ordered, the same pattern repeating throughout the object; in carbon, on the other hand, they are disordered. A carefully applied blow divides the crystal perfectly, but a blow on a glass causes it to break into a thousand pieces.

Not everything that glitters is gold, says a saying, we could also say that not everything that is transparent is glass. There we have the humble glass of windows, glasses and decorative figures, although transparent is not a crystal. On the other hand, not all glass is transparent, most metals in their natural state are presented as crystals and are opaque.

Materials in the vitreous state are by far the most abundant in nature and therefore are they cheap? On the other hand, materials in the crystalline state are much less abundant and therefore more expensive? Curiously, depending on their use in some occasions, glasses have advantages over crystals. For example, medicines need to be vitreous to be disposed of faster, which would not be the case if they were in crystalline state. At other times, the properties of crystals are needed. The silicon chips that dominate electronics are made from a silicon crystal.

How is a glass or crystal obtained? Both are solids and can be obtained by cooling from a liquid precursor. At high temperature, the motion of the atoms or molecules, hereafter particles, is characterized by continuous changes of direction, it is random, it is like that of a drunkard. The particles are free and move throughout the available space. As the liquid cools the movements become slower and slower, nuclei are formed and then these grow until all the particles are part of the solid structure, the particles are now confined and only vibrate. If the cooling is fast the end result is a glass, the particles do not settle into minimum energy arrangements, they do not have time to do so. On the other hand, if the cooling is slow, the result is a crystal, the atoms are arranged in positions of minimum energy, which happens in ordered arrangements. Or in other words, with calm we do things better.

Indirect methods are currently used to study these systems, methods of electromagnetic wave scattering, in particular X-rays. That is, we send projectiles to the material and study what results. Although current studies have produced great advances in the understanding of glasses and crystals, there are still unknowns to be solved. For example, in the case of crystallization, it is still debated whether the classical nucleation theory is an adequate theory to describe the nucleation process or whether more refined theories should be proposed.

The science behind glass and crystals

How then to study glasses and crystals? Directly studying the solidification process is impossible given the technical limitations to follow all the particles that make up the systems with sufficient spatial and temporal resolution.

In this context, we have recently presented in the literature a model of non-vibrational granular matter that has been shown to be successful in describing some processes in the glass transition and crystallization. The system consists of 1 mm diameter particles arranged in an observation cell that can be flat or curved. Alternating fields generated by electromagnets, controlled by a computer, are used to fluidize the particles and make their motion erratic.

In our system the particle dynamics is slow and the required spatial resolution is accessible by means of a standard video camera. The particle motion can be easily controlled and can range from diffusive to subdiffusive by controlling the characteristics of the applied field, such as frequency and amplitude.

In the case of crystallization we have found that the nucleation process is carried out by at least two steps. In a first step a stable amorphous aggregate is formed. In a second step, a rearrangement process begins within this aggregate, which leads to the formation of the nucleus. The growth process takes place at the periphery of the aggregate while simultaneously the ordered interior continues to grow. With these experiments we provide experimental evidence in support of non-classical nucleation theories.

In the case of the glass transition we have found that the aging process, where the glass continues to evolve and thus change, does not continue indefinitely and is frustrated by the formation of more compact and stable aggregates.

We have found that the non-vibrated granular system is an excellent tool for modeling processes occurring in systems consisting of very small and fast moving particles. The system is very versatile and can be modified to study various other processes and systems such as gelation, melting and active matter. In short, we use an inexpensive experiment to generate expensive science.