Fast vs slow water: explaining the fragile-to-strong transition

When water is cooled, the rate of increment of its viscosity achieves a greatest at a specific low temperature.

Illustration of water's local structures. The blue lines show the H2O molecules of tetrahedral structure, the red lines show H2O molecules of disordered structure. The large balls show oxygen atoms, the small balls show hydrogen and the dotted white lines represent Hydrogen bonds. Credit: 2018 Hajime Tanaka, Institute of Industrial Science, The University of Tokyo
Illustration of water's local structures. The blue lines show the H2O molecules of tetrahedral structure, the red lines show H2O molecules of disordered structure. The large balls show oxygen atoms, the small balls show hydrogen and the dotted white lines represent Hydrogen bonds. Credit: 2018 Hajime Tanaka, Institute of Industrial Science, The University of Tokyo

A Japanese research group driven by The University of Tokyo examined the fragile-to-strong transition of water. Not at all like most fluids, when water is cooled, the rate of increment of its viscosity achieves a greatest at a specific low temperature.

The group demonstrated that displaying water as a temperature-subordinate mixture of two states—disordered fast water and locally ordered slow water—disclosed the fragile-to-strong transition and avoided the faulty forecasts of past hypotheses in view of glassy behavior.

Water poses many behaviors. A portion of its chemical characteristics is well-known, for example, extending when it freezes to ice. A lesser-known interest, which it imparts to only a couple of different liquids, is the delicate to-solid change.

Clarifying this conduct, which identifies with how cool water flows, has for some time been a wellspring of discussion. Now, a persuading clarification has been advanced by scientists in Tokyo.

At the point when liquids cool, their dynamics slow down and they end up becoming viscous. For most liquids, the rate of slowing is consistent as an element of temperature and these are known as strong liquids.

For fragile liquids, be that as it may, the rate persistently increases as the temperature drops. Water is uncommon in such manner—it is fragile at room temperature, yet strong at low temperatures, where its rate of expanding thickness hits a pinnacle.

This fragile-to-strong change is subtle, happening just in the supercooled administration, beneath water’s standard the point of freezing. Early models attempted to interface it to glassy dynamics, as supercooled water is known to be a glass-former.

In any case, a group driven by The University of Tokyo’s Institute of Industrial Science (IIS) proposes a two-state hypothesis, as a result demonstrating water as a mixture of two existing liquids.

Mathematically, the strong/fragile distinction rests on the Arrhenius law for dynamical processes—strong liquids obey this law, but for fragile ones, the rapid increase of viscosity is super-Arrhenius. Scientists amended this view by considering water to consist of two states, termed “fast” and “slow,” which are structurally different, but both obey Arrhenius dynamics.

Study co-author Rui Shi said, “We simulated water by molecular dynamics and looked for structural patterns. H2O molecules always assemble into tetrahedral, but we saw that some of these local structures were highly ordered, others less so. The disordered states correspond to fast water, and dominate at high temperature, while the well-ordered slow state takes over as the sample cools.”

The equations derived from the two-state model successfully predict the fragile-to-strong crossover. This happens well above the glass transition point—glassy behavior seems to be a red herring as far as this issue is concerned.

The fact that fast water has Arrhenius, rather than power-law, dynamics also resolves faulty predictions based on earlier attempts to link water’s fragility to certain aspects of its phase diagram.

Lead author Hajime Tanaka said, “Fragile water may be an illusion. The apparent transition is an artifact of the temperature-dependent balance of two strong liquid states. The presence of two states reflects water’s tendency to form local structures, which is easier at low temperature. In fact, other liquids with a fragile-to-strong transition, like silica, also show local ordering. We propose that this, rather than any glassy behavior, is what distinguishes them from true fragile liquids.”

The study is reported in the journal PNAS.