Microscopic view of atomic interactions in molten glass.

Unlocking the Secrets of Glass: How Atomic Forces Shape Viscosity and Fragility

Rhea Morgan in Science & Nature August 2025 • 5 min read.

"New research questions conventional models of glass formation, revealing how interatomic repulsion and density scaling affect viscosity and fragility in glassy materials. Discover the implications for material science and technology."

The study of glass, a material ubiquitous in our daily lives, continues to present intriguing challenges to scientists. Unlike crystalline solids with their orderly atomic arrangements, glass possesses an amorphous structure, leading to unique and often unpredictable properties. A recent paper by Krausser et al. [1] has stirred debate within the scientific community by proposing a new perspective on the factors governing the isobaric fragility of glass-forming systems, linking it to thermal expansion and interatomic repulsion.

The central argument put forth by Krausser and colleagues suggests that, in metallic glasses, thermal expansion behaves largely independently of composition and shows little correlation with interatomic repulsion. As a result, the fragility of the glass increases with the increasing steepness of interatomic forces. This contrasts sharply with Lennard-Jones glasses, where a strong correlation exists between thermal expansion and interatomic repulsion, causing fragility to decrease as repulsion steepness increases.

These conclusions stem from a viscosity model that integrates the showing model of glass transition with the atomic theory of elasticity. However, a new commentary raises critical questions about the consistency of this model, particularly concerning its compatibility with established experimental observations, such as the power-density scaling rule. This rule, verified across numerous liquids and polymers, offers a framework for understanding how transport and relaxation properties scale with density and temperature.

The Density Scaling Debate

Microscopic view of atomic interactions in molten glass.

The heart of the controversy lies in how well the proposed model aligns with the empirically supported concept of density scaling. Density scaling posits that various transport and relaxation properties such as viscosity, structural relaxation time, and diffusion constant can be collapsed onto a single master curve when plotted against a scaled variable that incorporates both temperature (T) and volume (V), TV^γ, where γ is a material-specific constant. This scaling behavior implies a fundamental relationship between density and temperature in determining the dynamics of glass-forming liquids.

The researchers highlight that density scaling is inherently model-independent; it relies solely on the analysis of experimental data without presupposing any specific theoretical framework. Therefore, any valid theoretical model for glass-forming liquids should be consistent with the observed density scaling behavior. The commentary argues that the model proposed by Krausser et al. falls short in this regard, leading to potential inconsistencies when compared with experimental results.

Inconsistency in Application: The main point of contention revolves around the application of the proposed model and its consistency with experimentally observed features of supercooled systems, especially concerning the power-density scaling rule verified for numerous liquids and polymers.
Mathematical Examination: A mathematical comparison reveals that the model's equation for isobaric fragility closely resembles that derived from density scaling, suggesting a direct relationship between parameters.
Material Constant: The model introduces a parameter λ, which, when linked to the density scaling exponent γ (where γ = λ + 2), should align with experimental data. However, challenges arise when using reported values of density scaling exponents for common glass formers.
Experimental Verification: The model's consistency is tested against experimental data for a canonical glass-forming liquid, ortho-terphenyl (OTP), to verify compliance and predictive power.

To further illustrate the point, the scientists delve into the mathematical implications of the two models. By comparing the equation for isobaric fragility derived by Krausser et al. with that obtained from density scaling, they reveal a direct correspondence. Specifically, they demonstrate that the parameter λ in Krausser's model is directly related to the density scaling exponent γ, with the relationship γ = λ + 2. This relationship suggests that the two models should, in principle, be compatible. However, the commentary points out that using experimentally determined values of γ for well-known glass formers does not yield a satisfactory description of the experimental data when plugged into Krausser's model.

Challenging the Model

In conclusion, while the work of Krausser et al. offers a valuable perspective on the factors influencing glass fragility, the commentary highlights potential inconsistencies with the well-established density scaling concept. This raises critical questions about the model's ability to accurately describe the dynamic properties of supercooled systems. Further research and refinement are needed to reconcile these discrepancies and develop a more comprehensive understanding of the intricate forces governing glass formation.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1103/physrevb.98.016201, Alternate LINK

Title: Comment On “Disentangling Interatomic Repulsion And Anharmonicity In The Viscosity And Fragility Of Glasses”

Journal: Physical Review B

Publisher: American Physical Society (APS)

Authors: K. Koperwas, M. Paluch

Published: 2018-07-17

Everything You Need To Know

According to Krausser et al.'s research, how do interatomic repulsion and thermal expansion affect the isobaric fragility differently in metallic glasses versus Lennard-Jones glasses?

The isobaric fragility of glass-forming systems is thought to be influenced by thermal expansion and interatomic repulsion. Krausser et al. propose that in metallic glasses, thermal expansion is independent of composition and has little correlation with interatomic repulsion, causing fragility to increase with the steepness of interatomic forces. This is different from Lennard-Jones glasses, where thermal expansion and interatomic repulsion are strongly correlated, leading to decreasing fragility as repulsion steepens.

What is density scaling, and why is it considered an important concept in the study of glass-forming liquids?

Density scaling is an empirically supported concept showing that transport and relaxation properties, like viscosity, structural relaxation time, and diffusion constant, can be collapsed onto a single master curve when plotted against a scaled variable incorporating both temperature (T) and volume (V), TV^γ. Here, γ is a material-specific constant. It's model-independent, relying on experimental data, and any theoretical model should align with it.

How does the parameter λ in Krausser et al.'s model relate to the density scaling exponent γ, and what challenges arise when using experimental values of γ in their model?

The model proposed by Krausser et al. introduces a parameter λ, which relates to the density scaling exponent γ through the equation γ = λ + 2. The commentary suggests that using experimental values of γ for common glass formers in Krausser's model doesn't accurately describe experimental data. This inconsistency raises questions about the model's compatibility with experimental results.

How was experimental data for ortho-terphenyl (OTP) used to test the consistency of the model proposed by Krausser et al., and what were the findings?

The commentary evaluates the consistency of the model proposed by Krausser et al. against experimental data for ortho-terphenyl (OTP), a canonical glass-forming liquid. This verification aims to check the model's compliance and predictive power in relation to established experimental observations and the power-density scaling rule. Challenges arise when the model's predictions are compared with actual experimental results for OTP.

What are the implications of the potential inconsistencies between the model proposed by Krausser et al. and the density scaling concept, and what further research is needed?

While the research by Krausser et al. offers a valuable perspective, the commentary identifies potential inconsistencies with the established density scaling concept. Reconciling these discrepancies is crucial for developing a comprehensive understanding of glass formation. Further investigation should refine the proposed model or explore alternative frameworks to better align theoretical predictions with experimental observations. If the inconsistencies remain unaddressed, the applicability of the model to different types of glass-forming systems may be limited.