Glass transition

Some liquids become rigid as they cool, without their molecules lining up into an orderly crystal. The molecules become too slow to rearrange on the timescale of an experiment. What makes that motion slow so dramatically?[1]

What microscopic process makes a supercooled liquid’s structural relaxation so slow, and is there an underlying thermodynamic transition? Structural relaxation is the rearrangement that allows the liquid to forget an earlier configuration. Cooling can lengthen its characteristic time by 12–15 orders of magnitude. A liquid that rearranges during a short experiment at one temperature may remain trapped in an unequilibrated structure during the same experiment at a lower temperature.

The observed glass transition is therefore tied to the available observation time and preparation history. It does not by itself demonstrate an ideal equilibrium transition. Proposed explanations must relate the slowdown to molecular structure or collective dynamics, despite the lack of a crystal-like ordering pattern.[1]

The slowdown is observed, but its microscopic cause remains disputed. We also do not know whether it points toward a deeper transition that a liquid would undergo if it could stay in equilibrium long enough.[1]

The 2026 perspective asks whether the distances over which molecular behavior is correlated grow without bound, whether an ideal-glass transition exists, and whether different liquids share a common response to small disturbances. Deeply cooled liquids are hard to equilibrate, so the states needed to discriminate explanations can be inaccessible on practical timescales.

Machine learning can predict aspects of motion from structure in selected systems. The 2025 roadmap still identifies transferability and physical interpretation as open issues. A prediction that works in one model is not yet a general explanation of glass formation.[1] [2]

A glass holds its shape without a crystal’s regular arrangement. Explaining that behavior would tell us how disordered molecules can collectively produce rigidity, connecting familiar materials with the physics of liquids and solids.[1]

Glass formation tests whether a shared physical account can explain very different disordered materials. It connects the motion of individual molecules with a collective change in how matter responds, without the visible ordering that explains a crystal.

The diversity of glass-forming liquids makes that connection demanding: a useful account should explain common behavior while retaining genuine chemical differences. The 2026 perspective treats this balance between universality and material specificity as a central reason to study the problem.[1]