Computer simulations examine effects of shear on medium-density amorphous ice

Water is ubiquitous and seemingly ordinary, possessing no distinct color or odor. Though we often take water for granted, it is by no means a simple substance. As a consequence of its chemical properties, H₂O is one of the most incredible substances, able to form into 20 known separate crystalline ice phases. Now researchers are seeking to expand that number even further.

Ingrid De Almeida Ribeiro, a postdoctoral researcher in chemistry, and her lab partners in the Molinero Research Group at the University of Utah’s department of chemistry have published a study in Proceedings of the National Academy of Sciences detailing their work advancing the science of amorphous ice using computer simulations.

Often characterized as glass, amorphous ice had long been known to appear in either a low-density amorphous (LDA) or high-density amorphous (HDA) state. A recent study demonstrated the existence of medium-density amorphous (MDA) ice through the application of ball milling. Ribeiro’s work expanded upon this by examining the consequences of shear in addition to other variables, including temperature and pressure.

Amorphous ice is distinguished from typical crystalline ice by its non-periodic atomic arrangement. It is still classified as a solid material, which can be alternatively described as “a liquid that has lost its ability to flow.”

“Think about walking into a movie theater. All the seats are lined up in perfect rows and columns. That’s like crystalline ice—atoms arranged in a structured, repeating pattern. Now, picture a music festival, people are just scattered everywhere—some packed closely together, others with more space between them, no clear arrangement. That’s like amorphous ice.” Ribeiro said.

“Now, picture a liquid where atoms move freely. If you were to freeze that disordered structure without allowing the atoms to rearrange into a crystal, you’d get a glass. It’s like a frozen snapshot of a liquid.”

The lack of order in amorphous ice creates different local environments, which is why LDA and HDA are so distinct. In LDA, each water molecule is surrounded by about four neighbors, giving it a density close to that of crystalline ice. In HDA, the water molecules pack more tightly, with five to six neighbors, increasing its density by 25%.

For comparison, liquid water sits between these two, with a density 9% higher than ice at ambient pressure. To compress liquid water to the density of HDA, you’d need an incredible 3,000 atmospheres of pressure—more than twice the pressure found at the bottom of the ocean.

What excites researchers like Ribeiro is the “chasm” between LDA and HDA that can now be “filled in” with new glasses produced by ball milling or shear techniques.

“Until now, it was believed that water could only exist in either low- or high-density amorphous forms, with nothing in between,” Ribeiro said. “Our study showed that shearing results in a rearrangement of the water molecules into new glasses that have densities between LDA and HDA.”

Her research was overseen by Valeria Molinero, a distinguished professor in the U’s Department of Chemistry and a leading ice researcher, and was conducted with other members of the Molinero group and scientists at Argonne National Laboratory in Illinois.

A central component of the study modeled what happens to amorphous ice when it undergoes “ball milling.” In this process, a sample of ice along with stainless steel balls are confined inside a sealed jar and kept at a temperature of 77 Kelvin, or -321°F. The vessel is vigorously shaken, subjecting the ice matter to compression, tension, and shear forces. This process was key in “unlocking” MDA ice and offers a new pathway for scientists to investigate the properties of water to an even greater extent than before.

At low temperatures, shearing prevents ice molecules from reordering themselves into more stable, crystalline structures. This creates an amorphous, glass-like substance after shearing. As a result, conventional isobaric (constant pressure) cooling methods cannot achieve MDA densities—but shear processes like ball milling can.

With support from the U’s Center for High Performance Computing, Molinero’s team conducted a series of shear deformation simulations. One simulation involved ice grains, containing up to 4 million water molecules through a shear rate of 100 million times per second. Through simulations like these, the research team found more avenues for synthesizing MDA—not just from crystalline ice, but also from amorphous forms like LDA and HDA.

These findings could help scientists better understand water under extreme conditions, including its formation in extraterrestrial space. They provide insights into ice formations and structures on far-off moons, such as Europa and Enceladus, which orbit Jupiter and Saturn, respectively. With low pressure and frigid temperatures, conditions on these moons are similar to those created in research labs. While our everyday experience with ice involves its crystalline form, the most common structure of water throughout the universe is amorphous ice.

These discoveries are even more exciting because they could have applications beyond water, extending to other materials with similar local structures, such as carbon, silica and silicon. Like water, these materials can exist in multiple distinct amorphous phases and are widely used in industrial and electronic applications.

“While ball milling has been actively used to process materials, this study is the first to provide a unified perspective that incorporates shear as a new controlling variable of the phase diagram of the substance—an approach that could be applied to many other materials,” Ribeiro said.