Glass inequality

Not all glasses are created equal. Commonly, we think of the "structure" of a glass as being indistinguishable from a liquid except it's locked into the solid state. However, a new study of "amorphous ice", which forms when water is cooled to very low temperatures, suggests that there is hidden order within this type of glass. Understanding such order and the concept of hyperuniformity might be exploited in several applications including improving the role of amorphous silicon in electronics.

Researchers from Princeton University and the City University of New York used computer simulations to reveal that the water molecules in amorphous ice have previously undetected order [Martelli et al., Phys Rev Lett, 119, 136002 (2017) DOI: 10.1103/PhysRevLett.119.136002]. The finding might help explain some of the anomalous properties of water.

"According to our results, these types of glass are not simply frozen liquids," explains Princeton's Fausto Martelli. "We are essentially saying that a notion that scientists have believed for many years is partially wrong."

Until this study, the rapid freezing of water, as might occur in the cold of outer space, leads to the formation of a solid form of water very different from the down-to-earth crystalline ice with which we are familiar under our feet in winter or in our drinks in summer. Amorphous ice lacks crystallinity and has been thought of as a glass and is the most common form of solid water across the universe. However, amorphous ice has now revealed itself to have disordered hyperuniformity wherein there is long-distance, but not short-range, order. Disordered hyperuniform materials are thus part crystalline, part like a liquid.

"The existence of these large-scale structural correlations has not been fully appreciated, and that is really what we wanted to address in this study," explains team member Salvatore Torquato who discovered hyperuniformity in 2003 alongside Frank Stillinger. "The information present in these systems is quite striking, and leads to completely new insights about materials," he adds.

To explore the internal structure of amorphous ices, Martelli used a computer model that tracks the behavior of over 8000 water molecules to simulate what would happen if he cooled the water down to about 80 degrees Kelvin. Under such conditions, the water molecules no longer diffuse nor rotate and a hyperuniform pattern emerges in the simulation. The simulation provides indirect support for one possible explanation for water's anomalous behavior in that it at such low temperatures it can exist as a high-density and a low-density liquid phase. The transition between the two remains to be observed nevertheless. 

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".