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.
