Can hot water really freeze faster than cold water? Known as the Mpemba effect, this counterintuitive phenomenon has intrigued scientists for over 2,300 years. From Aristotle's early records to Mpemba's discovery in 1960 that hot milk freezes into ice cream more quickly, the Mpemba effect has remained controversial and its underlying mechanism remains unclear.

Seeking a more controllable system, researchers turned to glass, whose aging process involves no phase transition interference. In a study published in Physical Review Letters on May 19, a research team led by Prof. WANG Junqiang from the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences has reported a Mpemba-like effect in glass aging.

Using high-precision, high-rate nanocalorimetry, the researchers measured enthalpy relaxation in three types of glass: metallic, polymer, and molecular. They found that after high-temperature pre-annealing, glasses initially in a higher-energy state aged more rapidly than those starting from a lower-energy state. This suggests that the Mpemba-like effect is universal across glasses with quite different chemical bonds.

Classical theories assume that glass aging follows a single relaxation process where a glass in a higher energy state takes longer to reach the same lower energy state. The Mpemba-like effect discovered in this work challenges this conventional view.

By analyzing the heat flow relaxation peaks, the researchers found that the accelerated aging originates from the reactivation of β relaxation with a lower energy barrier during two‑step annealing. A higher β relaxation enthalpy leads to a faster aging rate, demonstrating that β relaxation plays a pivotal role in glass equilibration.

Interestingly, the Mpemba-like effect and the well-known Kovacs memory effect in glass both involve two-step annealing, but with opposite temperature sequences. This reveals a "non-commutative" property of annealing temperatures and highlights the complex interactions between different relaxation modes.

The study offers new insights for controlling glass energy and optimizing thermal treatment and relaxation processes.

This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, the Youth Science and Technology Innovation Leading Talent Project of Ningbo, and the Ningbo Major Research and Development Plan Project.

The enthalpy relaxation processes of glasses (Image by NIMTE)