2023-08-27 01:44:09
Glass may look like a perfectly ordered solid, but up close its chaotic arrangement of particles resembles the turbulent chaos of a free-falling liquid that solidifies over time. The materials in this state, known as amorphous solids, defy simple explanation. New research involving computation and simulation yields clues. In particular, it suggests that somewhere between the liquid and solid states a kind of rearrangement is occurring that we didn’t know existed. According to scientists Dimitrios Vragakis, Mohamed Hashim, and Kranthi Mandadabo of the University of California, Berkeley, there is a temperature boundary behavior for supercooled liquids and solids, in which static particles remain excited and “shrink” in place. We are broadly aware of three basic states of matter in everyday life: solid, liquid, and gas or vapor. Each is defined by the relationships between its molecules and their environment. When one of these elements transforms into another—the melting of a solid into a liquid or the vaporization of a liquid into a gas, for example—it’s called a metamorphosis. But matter is much more complex than these three basic states. The atoms can get so hot that their charges separate to form a plasma. When cooled, certain classes of particles can completely lose their identity and merge into a quantum blur. border frame = “0” allow = “accelerometer; Auto read write to clipboard. encrypted media; gyroscope; picture in picture; Web share” allows full screen > Amorphous solids are curious mixtures of well-ordered solids and liquids that are not bound together. Whereas particles within solids tend to form predictable connections with their neighbors once they have settled down to low enough temperatures, amorphous solids have an irregular arrangement of a liquid. How these seemingly random bonds transition from viscous streams of fluid molecules to a static landscape is far from clear. Using glass as the most common example, the building blocks of oxygen and silicon flow when heated. When cooled slowly, these particles have time to form into an organized crystalline structure called quartz. If cooled quickly, the particles somehow retain a disordered arrangement; This is the point at which it becomes an amorphous solid, and the temperature at which this occurs is the starting temperature. Vragidakis, Hasim, and Mandadapo used calculation and simulation, along with the results of previous experiments, to determine that this transition may not be so sharp, as it is characterized by peculiar activity of particles between their normal liquid states and their supercooled states. “Our theory predicts the initial temperature measured in model systems and explains why supercooled liquids behave around this temperature similar to solids, even though their structure is the same as that of a liquid.” Mandabo explains. “The initiation temperature of glass dynamics is similar to the melting temperature that ‘melts’ a supercooled liquid into a liquid. This should be appropriate for all supercooled fluids or glass systems.” The yellow regions show more moving molecules above their initial temperature (left), which transition to a more solid state, shown in blue, when they are very cold (right). (Kranthi Mandadapu) Although the total flux of atoms in the supercooled liquid is virtually zero, the particles are constantly changing in composition when they are stuck in place, leading to motions called excitations. The researchers manipulate these excitations in a supercooled 2D liquid, such as defects in a crystalline solid, and calculate what happens when the temperature changes. They discovered that the associated excitation pairs loosen at the initial temperature, causing the material to lose its solidity and behave like a normal liquid. The team believes their model can be extended to also understand how transition works in three dimensions and provide a theoretical basis for future experimental work. “The whole endeavor is to understand under a microscope what separates a supercooled liquid from a liquid at a high temperature,” says Mandabo. “It’s fascinating from a basic science perspective to study why these supercooled fluids exhibit markedly different dynamics than the normal fluids we know.” The research was published in the Proceedings of the National Academy of Sciences.
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