When liquid cools, its random molecular agitation weakens. Below a specific temperature, solidification results in crystal formation with a well-ordered and highly-rigid structure; or the liquid enters an increasingly viscous state, which leads to the formation of glass. Glasses are as stiff as crystals, and yet their spatial organisation has no apparent order! Is there a "hidden order" in glasses? This debate has been raging for several decades and is extremely difficult to settle experimentally. According to some theorists, a period of time equivalent to the age of the Universe would be required before this order is established in a domain with a size of ten molecular diameters! In other words, probing the system in a state where order has been sufficiently developed to ascertain immediate experimental evidence, is a "mission impossible" within a human lifetime.
Physicians had the idea of overcoming this difficulty by using a highly generalized property of "critical" phenomena, related to phase transitions, such as liquid-solid transformations. Any emerging order during transitions is always accompanied with criticality (very sharp increase) of the response of the material to external forces, such as an electrical field. It is thus by measuring the non-linear response (from 3 to 5) of a glass material (as a function of temperature and of the frequency of the applied electrical field) that the researchers were able to highlight the signs of the desired transition and link them to an order qualified paradoxically as a "amorphous structure".
They also demonstrated that the initiated order corresponds with a collective energy optimization, but no spatial uniformity is produced in the molecular arrangement. The experiments show that the "amorphous" size of the domain increases as and when the liquid temperature is lowered and when the "glass transition" phase is approaching.
Two entirely separate experiments were conducted: one in Saclay, the other in Augsbourg. The consistent results obtained explicitly prove the existence of amorphous order in glass. They overturn some theoretical approaches that describe glasses as simple ultra-viscous liquids. However, several theories remain in the running to explain the root cause of the highlighted transition phase. The researchers will now strive to test them, using new ground-breaking experiments.
This result, published in Science, is a major breakthrough with regard to international research partnerships funded by the Simons Foundation, which are striving to express the state of material in vitreous material as an equation. Giulio Biroli from the Institute de physique théorique (IPHT, CEA-CNRS, Saclay) is involved in these partnerships.