Snapshot

Coldwater Split

By Jason Warshof

Type “water anomalies” into Google, and your first hit will be a website that describes 64 unusual characteristics of the Earth’s most common liquid. You’ll read about dielectric constants and coefficients of expansion, diffracted x-rays and NMR spin lattice relaxation.

But to understand what’s so strange about water, we need only turn to the experience of a five-year-old child, says H. Eugene Stanley, a University Professor of physics, chemistry, biomedical engineering, and professor of physiology in the School of Medicine. Hand that child a glass of water in which ice cubes sank to the bottom, and she would likely exclaim, “What’s wrong with this?” Likewise, many of us take for granted one of water’s most basic peculiarities—that it becomes less dense when it freezes, a property shared by very few other liquids.

Stanley has always been fascinated by water’s quirky characteristics, and research he conducted with the help of two former graduate students has led to a groundbreaking discovery: liquid water can actually exist in two different states, low- and high-density. At temperatures below about –50°C, the molecular structure of liquid water moves from a state of loose dynamism to one of increased rigidity.

Few stories of scientific accomplishment happen overnight, and Stanley’s is no exception. His first major breakthrough came in 1992 when—working with then–graduate student Peter Poole—he was the first to show, through computer simulations, that liquid water at very low temperatures could exist in both high- and low-density states. The resulting paper was a sensation and has been cited in more than 450 peer-reviewed publications, evidence of its transformational effect.

“It’s one thing to take part in a discovery as a faculty member,” says Poole, “but it’s a particular thrill to do it as a graduate student.”

Thirteen years passed before Stanley’s theory of two states of liquid water—which was based on computer data—would be verified by means of actual experimentation.

While gathering important experimental data on the so-called phase transitions of cold (or “supercooled”) liquid water, a team at MIT led by nuclear engineer Sow-Hsin Chen faced the tough task of interpreting their findings. That’s where the theoretical work, again based on computer simulations, of Stanley and graduate student Limei Xu became especially useful. Their illuminating results on water on the “warm” side of the critical point at which it changes continuously from a low- to a high-density-like liquid helped explain the MIT researchers’ experimental results, in essence confirming Poole and Stanley’s initial hypothesis.

“For me,” says Xu, “this was a chance to see the effect of real experiments, not just the pure theoretical stuff I’d been working on.”

While it’s still early to begin identifying concrete applications, the implications of liquid water’s two phases are vast. Already, scientists in such fields as biology, chemistry, and materials engineering have shown interest in exploring the effects of supercooled water on DNA, proteins, and other substances—which could lead, for instance, to enhanced methods of preserving sperm, oocytes (female reproductive cells), and other biological matter. Researchers are far from having a comprehensive understanding of the human body, and water always figures in the equation. “Water is ubiquitous,” says Stanley. “We’ll really never understand life until we understand water.”