Scientific discovery sheds light on the mystery of the glass passage

Researchers at the School of Chemistry at Tel Aviv University have developed a new experimental and theoretical method for monitoring the transition from a flowing liquid to an amorphous solid, using tiny marker particles that reveal how the material loses its ability to transfer momentum and approaches a glassy state.

A scientific discovery by the School of Chemistry at Tel Aviv University offers a new perspective on the scientific mystery – how does a flowing liquid suddenly turn into a solid, almost frozen substance, without changing its structure? This question, known as the “glass transition,” has preoccupied physicists for more than a century. The research offers a new experimental and theoretical way to observe the elusive process – by tracking the movement of tiny particles that serve as “sensors” within the material.

The study was conducted by Prof. Haim Diment and Prof. Yael Roychman from the School of Chemistry at Tel Aviv University, together with the research group of Prof. Stefan Egelhoff from the University of Düsseldorf, and was published in the journal Nature Physics.

The research deals with colloidal materials – suspensions of microscopic particles in a liquid – which are considered an ideal model for studying the glass transition. When the particle concentration is low, the system behaves like a normal liquid. But as the density increases, the particles restrict each other’s movement, until the entire system “gets stuck” and takes on the properties of an amorphous solid, similar to glass.

The researchers' key innovation is the use of extremely small, mobile particles, integrated into a system of larger particles that undergo the glass transition. While the larger particles gradually lose their ability to move, the smaller particles continue to move – thus making it possible to measure how the medium around them changes.

Using advanced microscopy, the coordinated motion of pairs of small particles was measured: how the motion of one affects the other, along different directions and at varying distances. The results paint a clear picture: When the system is a liquid, the motion "propagates" over large distances through the liquid. But as you approach the glassy state, this propagation slows down, and the system begins to behave like a solid, absorbing momentum instead of transmitting it.

The researchers identified three distinct signs of the transition: a sharp change in the way the correlation decays with distance, the appearance of a new characteristic length that increases with the viscosity of the material, and even opposing motions between adjacent particles—evidence of the formation of shear resistance, a fundamental property of solids. The experimental results confirmed with great accuracy theoretical predictions made by the same team several years ago.

The team of researchers notes that beyond their importance for a deeper understanding of the glass transition, the findings have broad implications. The new method could be used to study gels, soft materials, active systems, and even biological tissues—areas where it is difficult to detect when a system stops “flowing” and starts to harden. In this sense, the tiny particles serve as microscopic witnesses to the moment when a liquid loses its character.

Prof. Haim Diment concludes: "The importance of the study is not only in identifying new signs of the glass transition, but in a new perspective on the entire phenomenon. According to him, the findings show that the glass transition is not limited to a gradual slowing down of particle motion, but is accompanied by a profound change in the way the material transfers momentum from point to point. The use of small marker particles as hydrodynamic probes opens up the possibility of examining the emergence of solid properties even before the system has actually stopped flowing, and may provide a new tool for studying soft materials and complex systems in which the transition from liquid to solid is difficult to measure."

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