Scientists theorize a hidden phase transition between liquid and a solid

Amorphous materials are anything composed of glass or plastic. The atoms and molecules in amorphous materials never stack up to form crystals when they are cooled, in contrast to many materials that do. Although glass and plastic are often thought of as "solids," they really remain in a condition that is better described as a supercooled liquid that flows very slowly.

Although these "glassy dynamic" materials are common in our daily lives, scientists have long been baffled by how they become stiff at the tiny level.

Now, scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab) of the Department of Energy have found molecular activity in supercooled liquids that is indicative of a covert phase change between a liquid and a solid.

Their expanded knowledge relates to common materials like plastic and glass and might aid researchers in the creation of novel amorphous materials for application in additive manufacturing, medical devices, and drug delivery.

The scientists specifically explained why the molecules in these materials, when cooled, remain disordered like a liquid until taking a sharp turn toward a solid-like state at a certain temperature known as the onset temperature—in effect becoming so viscous that they barely move. What distinguishes supercooled liquids from ordinary liquids is this commencement of stiffness, a hitherto unrecognized phase change.

The research, which was published in PNAS, was led by Kranthi Mandadapu, a staff scientist in Berkeley Lab's Chemical Sciences Division and professor of chemical engineering at the University of California, Berkeley. "Our theory predicts the onset temperature measured in model systems and explains why the behavior of supercooled liquids around that temperature is reminiscent of solids even though their structure is the same as that of the liquid," she said.

Any supercooled liquid alternates between various molecular arrangements constantly, causing excitations, or localized particle motions. The excitations in a 2D supercooled liquid were handled as flaws in a crystalline solid in Mandadapu, postdoctoral researcher Dimitrios Fraggedakis, and graduate student Muhammad Hasyim's suggested theory.

They suggest that every occurrence of a bound pair of flaws split apart into an unbounded pair when the temperature of the supercooled liquid rose to the onset temperature. It was precisely at this temperature when faults unbound, causing the system to lose stiffness and behave more like a typical liquid.

"The starting temperature for glassy dynamics is analogous to the melting point at which a supercooled liquid'melts' into a liquid. All supercooled liquids or glassy systems should be affected by this, according to Mandadapu.

Other crucial aspects of glassy dynamics were also captured by theory and simulations, such as the finding that only a small number of particles moved for brief periods of time while the majority of the liquid stayed frozen.

The goal, according to Mandadapu, is to comprehend at the microscopic level what distinguishes a supercooled liquid from a high temperature liquid.

Theoretically, Mandadapu and his coworkers can adapt their model to 3D systems. Additionally, they want to develop it to clarify how localized movements trigger neighboring excitations that in turn cause the liquid as a whole to relax. Together, these elements could offer a dependable microscopic view of how glassy dynamics evolve in a way that is compatible with cutting-edge data.

Examining the reasons behind why these supercooled liquids display significantly different dynamics from the usual liquids that we are familiar with is intriguing from a basic science perspective, according to Mandadapu.