Breakthrough Study Resolves Century-Old Debate: Physicists Prove Glass Can Exist in Thermodynamic Equilibrium

In a groundbreaking study that upends more than a century of physics assumptions, researchers at Utrecht University have experimentally demonstrated that glass can exist in a stable thermodynamic equilibrium—a disordered state where particles are frozen in place but can still rotate. Published in Nature Communications in March 2026, the study resolves a fundamental debate: whether glass is a solid or an ultra-slow-moving liquid, while introducing a new framework for understanding disordered materials. The findings suggest that particle shape plays a pivotal role in glass formation, a factor long overlooked in classical theories.

The Centuries-Old Glass Conundrum: From Medieval Windows to Modern Physics

For generations, the idea that glass might behave like an extremely slow liquid has captivated both scientists and public imagination. The myth of medieval cathedral windows thickening at the bottom due to centuries of imperceptible flow persists despite being debunked by materials scientists. Historical records reveal that these uneven panes resulted from the manual glassmaking techniques of the era, where molten glass was unevenly spread and installed with the thicker portion at the bottom for structural stability. Yet the question remained: if not from flow, then what *is* glass? The answer, scientists now confirm, lies in its intrinsic disorder.

Why Glass Defies Simple Classification

Unlike crystalline solids with their orderly atomic lattices or liquids where molecules move freely, glass occupies an ambiguous middle ground. Its atoms or molecules are locked in a rigid arrangement without long-range order, a state known as an amorphous solid. Traditional textbooks define glass as a ‘supercooled liquid’—a material cooled too rapidly to crystallize, freezing its atoms in a chaotic, non-equilibrium state. According to classical thermodynamics, such a state should be unstable over time, slowly evolving toward a lower-energy crystalline form. But experimental evidence has never supported this inevitability—until now.

A New Paradigm: Equilibrium Glass States and Rod-Shaped Particles

Led by physicist Thijs Besseling, the Utrecht team constructed a colloidal model system using rod-shaped particles—microscopic rods suspended in a fluid—rather than atomic-scale glass. Colloids, which exhibit macroscopic behavior analogous to atomic systems, allow researchers to observe particle dynamics under a microscope, a feat impossible with atoms. While spherical colloids have been used extensively in glass studies, rod-shaped particles introduce a critical new variable: rotational freedom.

The Experiment: Freezing Motion While Allowing Rotation

In their experiment, the researchers increased the density of the rod-shaped colloids until the particles could no longer translate (move sideways) but remained free to rotate. What emerged was a disordered, glass-like state that persisted indefinitely—and crucially—was energetically favorable. When subjected to an external electric field designed to push the system toward a crystalline state, the rods resisted transformation. As soon as the field was removed, the system reverted to its glassy equilibrium. "The claim that a glass-like state can be in equilibrium is quite controversial," Besseling noted. "This behavior contradicts the long-standing assumption that glasses are inherently out of equilibrium."

“A glass and an equilibrium state exclude each other in many people's minds. But our results show that under specific conditions, a disordered state can be the most stable configuration—even more stable than a crystal.” — Thijs Besseling, Utrecht University physicist and lead author

Computer Simulations Validate the Breakthrough

To confirm their findings, the Utrecht team collaborated with Laura Filion, a computational physicist at Utrecht, who ran simulations of a simplified colloidal model. The results mirrored the experimental observations: at high particle densities, the disordered state was more energetically stable than the crystalline arrangement. "We see it both in a complex experiment and in a simple computer model. That gives us confidence in the results," Filion stated. The convergence of experimental and theoretical evidence strengthens the claim that equilibrium glasses are not only possible but may be common in systems where particle rotation is significant.

Why This Discovery Could Change Materials Science Forever

The implications of this research extend far beyond academic curiosity. A deeper understanding of the glass transition—the point at which a liquid becomes a rigid, disordered solid—could revolutionize how we design and manufacture a wide array of materials. Current theories of glass formation do not account for particle shape or rotational dynamics, both of which this study identifies as crucial factors. Materials like metallic glasses, polymer coatings, and even biological tissues often exhibit glass-like behavior. By incorporating rotational degrees of freedom into models, scientists may finally be able to predict the lifespan, durability, and performance of disordered materials with unprecedented accuracy.

From Batteries to Biological Systems: Real-World Applications

The potential applications are vast. In energy storage, glassy electrolytes in solid-state batteries could benefit from models that account for their amorphous structure and stability. Coatings used in aerospace and electronics often rely on glassy polymers; understanding their long-term stability could reduce failures and extend product lifespans. Even in biology, cellular membranes and cytoskeletal networks display glass-like behaviors during processes like growth and repair. The principles uncovered in this study may help explain how these biological systems maintain order within apparent disorder.

Challenging 100 Years of Glass Theory

For over a century, the scientific consensus held that all glasses are inherently out of equilibrium and will eventually crystallize given enough time—a process known as devitrification. This belief stemmed from the second law of thermodynamics, which favors lower-energy states. Yet, in the natural world, many glasses—such as volcanic obsidian—remain in a disordered state for millennia without crystallizing. The Utrecht study offers a plausible explanation: particle shape and rotational freedom can stabilize disordered states, creating equilibrium glasses that defy classical expectations. "This work doesn't invalidate the idea of supercooled liquids," says Dr. Peter Wolynes, a theoretical physicist at Rice University not involved in the study, "but it shows that the landscape of possible glassy states is far richer than we previously imagined."

What This Means for Real Glass—and Beyond

Importantly, the study does not prove that everyday window glass is in equilibrium. Atomic systems behave differently from colloidal ones due to stronger interparticle forces and quantum effects. However, the principle that particle geometry influences glass formation is now firmly established. Many molecules—including proteins, polymers, and liquid crystals—are not spherical. Their elongated or asymmetric shapes may allow rotational degrees of freedom that stabilize glassy phases. This insight could lead to the development of new materials engineered at the molecular level to resist crystallization or tailor their mechanical properties.

• Physicists at Utrecht University have experimentally created an equilibrium glassy state using rod-shaped colloidal particles, challenging classical thermodynamics.
• The breakthrough shows that particle shape and rotational freedom can stabilize disordered states, offering new insights into glass formation.
• The discovery could accelerate innovation in materials science, including batteries, coatings, and biomaterials.
• Computer simulations confirmed the experimental results, providing strong evidence for the existence of equilibrium glasses.
• This research redefines the glass transition, potentially resolving long-standing debates in condensed matter physics.

A Paradigm Shift in Condensed Matter Physics

The Utrecht study represents a rare moment where experimental observation precedes theoretical revision. Historically, advances in glass physics have been incremental, with most progress coming from refining existing models. But this work introduces a new class of disordered states—equilibrium glasses—that were previously thought impossible. It invites a reevaluation of how we classify amorphous materials and opens the door to exploring similar phenomena in other complex systems, such as foams, gels, and granular materials. As Filion notes, "We’re not just adding a footnote to glass theory—we’re redrawing the map."

The Role of Colloids in Modern Materials Research

Colloidal systems have become indispensable in studying phase transitions and material behavior. Unlike atoms, colloids can be directly observed and manipulated, making them ideal for testing theoretical predictions. The use of rod-shaped colloids in this study highlights the growing importance of anisotropy—directional dependence—in materials design. Such particles are already used in liquid crystal displays, drug delivery systems, and advanced filtration technologies. This research may inspire new colloidal applications where controlled disorder and stability are desired.

Future Directions: From Lab to Industry

While the immediate impact is theoretical, the long-term consequences could be transformative. Researchers are now exploring whether equilibrium glasses can be formed in other anisotropic particle systems, such as ellipsoidal colloids or nanorods. Industrial applications could include the development of ultra-stable glasses for optical devices, scratch-resistant coatings, or even self-healing materials. The Utrecht team plans to extend their work by investigating how external forces—such as shear or magnetic fields—affect the stability of equilibrium glass states. Such studies could pave the way for smart materials that adapt their structure in response to environmental changes.

A Call for a New Physics of Disorder

The equilibrium glass discovery underscores a broader trend in physics: the need to move beyond idealized models and embrace complexity. Real-world materials rarely conform to perfect crystals or pure liquids. The Utrecht study is a reminder that nature thrives in disordered states—and that our understanding of matter must evolve accordingly. As Besseling reflects, "We’ve spent a century trying to fit glass into a box. It’s time we acknowledged that the box itself was too small."