For centuries, researchers have tried to understand how glass can behave as a rigid solid while its molecules remain arranged in a disordered, liquidlike fashion. Conventional glass forms when a molten material cools so quickly that its molecules become locked into place before they can crystallize, producing a mechanically stable but amorphous structure. This unusual combination of disorder and rigidity underlies the optical clarity and mechanical properties that make glass indispensable for windows, bottles, phone screens and many other technologies.
To physicists, however, glass is more than everyday silica-based products. The term encompasses any solid whose molecules form a frozen, amorphous configuration rather than an ordered crystal, including many plastics, metallic glasses and some biological materials. In the University of Oregon lab, researchers focus on how molecular shape and packing control the properties of such materials, seeking general rules that link microscopic structure to mechanical behavior.
UO physicist Eric Corwin and his colleagues set out to address a challenge first articulated in 1948 by Princeton University chemist Walter Kauzmann. Kauzmann theorized that if a glass could be cooled or otherwise brought to an extremely low energy state, its molecules would reach an ideal configuration where they are as densely packed as possible while remaining amorphous. In this ideal glass, the material would mechanically resemble a crystal, with properties such as higher melting temperature, greater resistance to breaking under stress and potentially enhanced flexibility.
Because no naturally occurring material reaches this limit, experimentalists have lacked a real world example to study. Corwin's group instead approached the problem computationally. They modeled a two dimensional system of molecules represented as round disks and then designed an algorithm to assemble these disks into maximally dense, mechanically stable arrangements. The goal was to realize Kauzmann's ideal state in silico and then test whether it behaves more like a crystal than a conventional glass.
The researchers drew inspiration from the structure of a perfect two dimensional crystal, where each disk touches six neighbors, forming a honeycomb like pattern that fills space efficiently. They then developed a way to preserve the perfect local packing of each disk against its neighbors while eliminating the long range periodic order that characterizes a crystal. The result is a structure in which every disk is tightly constrained, yet the overall pattern lacks repetition and remains amorphous.
According to the team, these modeled configurations represent the densest possible packings for a given set of disks that do not revert to a crystalline lattice. To verify that they had indeed reached an ideal glass state, the researchers compared the simulated material's response to mechanical and thermal stresses with known properties of crystalline solids. They examined how the structure responded to pressure, bending and melting like conditions and found that its behavior closely matched that of a crystal, despite its disordered arrangement.
"The conclusion is that our structure mechanically behaves identically to a crystal, even though it is completely amorphous," Corwin said. The result provides the first concrete example of Kauzmann's ideal glass and offers a new platform for exploring how stability can emerge in systems that lack conventional order. It also supplies a benchmark for testing theories of the glass transition, the poorly understood transformation in which a liquid becomes a glass as it cools and its molecules fall out of equilibrium.
The work has potential implications for engineering new materials with tailored combinations of strength, flexibility and thermal resistance. Understanding how to design amorphous structures that approach ideal packing could guide the development of glasses capable of withstanding high temperatures and pressures or resisting fracture in demanding environments. Such materials could find uses in high performance applications ranging from precision optics to industrial components.
Corwin and his team are now extending their approach from two dimensional disk packings to fully three dimensional systems more representative of real materials. One promising target is metallic glass, a class of alloys whose atoms form a disordered solid rather than a regular crystalline lattice. Metallic glasses can be exceptionally strong and resistant to deformation and can be processed using methods such as injection molding, but they are notoriously difficult to produce because they must be cooled extremely rapidly from the melt to avoid crystallization.
A better theoretical grasp of the glass transition and of what makes an alloy more or less amenable to glass formation could expand the range of metallic glasses that can be produced. Corwin suggests that being able to cool such materials more slowly without crystallization would be transformative for manufacturing. With sufficiently robust metallic glasses, industries might one day mold complex components like car engines or jet aircraft parts directly from the melt, radically changing how these systems are designed and produced.
Research Report:Ideal Glass and Ideal Disk Packing in Two Dimensions
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