Rethinking the path through water ‘no man’s land’

A century ago, “no man’s land” described the barrier between troops in World War I. Today, it describes a barrier for scientists: Between 150 and 220 Kelvin, supercooled water crystallizes within a fraction of a microsecond, too fast for standard observational techniques to explore.

Previous research probed this elusive zone with lasers by heating low-density amorphous ice using pulsed-heating procedures (PHP). Such methods enabled samples to persist in a supercooled liquid state for nanoseconds before rapidly cooling back to solid amorphous glass. Ingrid de Almeida Ribeiro and Valeria Molinero conducted molecular dynamics simulations to determine that the liquid molecules’ structural relaxation during PHP was not proportional to their diffusion. Accurately determining diffusion is essential to determining if water in no man’s land is in a metastable supercooled state.

“We cannot conclude from those experiments, even though they are the best ones, the equilibrium dynamics of water in no man’s land,” said Molinero. “The story is not finished. There is more to learn.”

The researchers simulated supercooled water under PHP to reveal that, as the temperature decreased in no man’s land, the molecules’ relaxation and diffusion rates increasingly diverged. They concluded that molecular diffusion occurs up to orders of magnitude slower than structural relaxation due to PHP cycles “tugging” the liquid, rather than equilibration.

Understanding the mobility and relaxation of water molecules in no man’s land and its relation to ice formation has important implications for cryopreserving tissues and cells and understanding water crystallization in high-altitude clouds.

“PHP measures something really interesting,” said Molinero. “But it does not directly measure the equilibrium dynamics of individual water molecules.”

Source: “Non-equilibrium relaxation in water no man’s land,” by Ingrid de Almeida Ribeiro and Valeria Molinero, Journal of Chemical Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0325450 .

This paper is part of the Festschrift in honor of Christoph Dellago: Exploring Paths and Barriers in Statistical Mechanics Collection, learn more here .