Water appears to have a secret liquid phase

Water is unlike any other substance we encounter daily. Imagine a frozen pond, its icy surface concealing a liquid world beneath, while storm clouds swirl overhead. This interplay of solid, liquid, and gas highlights water’s remarkable versatility.

Not to mention, water is one of the few substances whose solid incarnation is less dense than its liquid form, explaining why ice cubes bob around your drink.

Does water have a second liquid phase?

Scientists from the University of California San Diego recently revealed another twist in water’s tale.

Under a specific set of conditions comprising high pressure and low temperature, water splits into two liquid phases, high-density and low-density. Once confirmed, this discovery could make a significant impact in the world of physics.

The research was led by Francesco Paesani, a professor of chemistry and biochemistry at UC San Diego.

Professor Paesani and his team create realistic molecular models that mirror experimental measurements, thanks to the application of machine learning techniques and algorithms.

“Our water model is so realistic you can almost drink it,” said Professor Paesani.

Water is not always homogenous

Typically, liquids are homogenous – they flow seamlessly, and distinguishing one molecule from the next would be impossible. This general rule applies to water as well.

However, in 1992, researchers theorized that beyond a certain temperature and pressure, water would no longer remain homogenous. Now, the UC San Diego team has conducted simulations that verified this theory.

The researchers pinpointed the critical point, approximately 198 Kelvin (-103 Fahrenheit) and 1,250 atmospheres, where water spontaneously separates into high-density and low-density phases.

Beyond this point, water alternates wildly between these phases; a phenomenon that defies expectations and evolves at a molecular level.

The 1992 researchers relied on a basic simulation to predict this unusual behavior in water. Since then, scientists have made several attempts to recreate this spontaneous division in controlled lab conditions, but without success.

Nevertheless, the world of computational modeling has evolved significantly over the past decades, especially with the rise of “data-driven many-body potentials.” This is a field that Professor Paesani’s team specializes in.

Water across the entire phase diagram

The data-driven many-body model of water (MB-pol) developed by the Paesani group is trained on high-level quantum mechanical calculations (data-driven) and rather than calculating the energy of an entire system at once, they deconstruct energy in terms of individual contributions (many-body).

According to the researchers, these reference energies are fed into a machine learning model that is then able to provide realistic simulations of water across the entire phase diagram.

Just as adding another person to a crowded room hardly affects the behavior of individuals, water molecules behave in a certain way until a point where additional changes make no significant impact, noted Professor Paesani.

“Quantum mechanical simulations can be extremely expensive. You might be able to calculate the energies of five or six water molecules. Our method, using MB-pol and machine learning, allows us to run simulations for up to several microseconds,” said Paesani.

“This is something computational molecular scientists have dreamed about for a long time.”

The road ahead: Liquid-liquid separation

Despite the significant breakthrough, the journey was far from easy. The simulations used in this research took around two years of continuous calculations, made possible only by some of the world’s most powerful supercomputers.

As we look forward, this research could potentially lead to the creation of synthetic liquids exhibiting similar liquid-liquid transitions as water under ordinary conditions.

Imagine porous liquids that behave akin to sponges and are capable of capturing pollutants or aiding in water desalinization.

“The simulation took almost two years, so this is a really exciting accomplishment,” said Paesani. “I think our estimate is very realistic. Now it’s up to the experimental researchers to prove our predictions right.”

Though recreating these conditions in a lab remains challenging, advances in nanodroplet technology – for instance, creating tiny water droplets that generate high internal pressure through surface tension – could lend a helping hand.

The full study was published in the journal Nature Physics.

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