Water wisdom and Widom Probably the most ubiquitous substance on our planet – and with a stunningly simple structure [two hydrogen atoms and an oxygen atom] – water may be among the most mysterious substances known. Anyone who has seen the results of pipes freezing in winter, or watched ice cubes float in their drink, has seen examples of water’s bizarre behavior. Rather than contracting and becoming denser when it freezes, water expands – becoming about 10% less dense as a solid than as a liquid. This allows fish and other aquatic creatures to live in the still-liquid water beneath the frozen winter ice of lakes and streams.
Gene Stanley and his students and colleagues at BU’s Center for Polymer Studies have long studied water and its oddities. They are part of a long line of scientists through the ages who have puzzled over the mysterious transitions of water as it moves from gas to liquid to solid and back again. One particular area of investigation at the Center has been supercooled water; water that exists in a liquid state well below 0ºC, the temperature at which water normally freezes. This particular property is crucial to plants ability to survive the winter and re-emerge in spring.
While a graduate student in Stanley’s lab, Peter Poole, now a professor at St. Francis Xavier University in Nova Scotia, pioneered the use of computer simulations to study the behavior of liquid water at very low temperatures, temperatures so low that physical experiments were virtually impossible. A series of investigators at the Center, beginning with Poole in 1992 and culminating with Giancarlo Franzese, Anna Skibinsky, and Sergey Buldyrev in 2005, were in fact able to demonstrate that liquid water has two distinct states, high- and low-density.
It has long been known that as pressure increases the temperature needed to condense steam into water increases (the gas/liquid transition). But, as Thomas Andrews discovered in 1869, above a certain temperature no matter how high the pressure, the water will remain in a gaseous state. This is known as the “critical point”.
Poole’s computer simulations predicted unusual behavior for supercooled water. He found that as water was cooled beyond the critical point a new line could be calculated to describe the behavior of the water molecules as they fluctuate between the two liquid states, on one side of the line the water would exist as a high density liquid, on the other side as a low density liquid. Stanley dubbed this the Widom Line, in honor of Benjamin Widom, a distinguished chemical physicist who first proposed that fluids fluctuate in strange ways near the critical point.
In an article that appeared in a recent issue of the Proceedings of the National Academy of Sciences (15 November 2005) Stanley, his graduate students, Limei Xu, Pradeep Kumar, and colleagues, Poole, F. Sciortino (Univerità di Roma), S.-H. Chen (MIT), and S.V. Bulyrev interpret results of an experiment performed in Chen’s laboratory. In it, water was confined in thin capillaries (with a diameter of less than 5 water molecules), cooled to a temperature of -73ºC, and bombarded with neutrons. The thinness of the capillaries kept ice crystals from forming, thus preventing freezing. The analysis showed that in this supercooled state a Widom line is generated. The line describes a temperature/pressure relationship boundary beyond the critical point, at which the high density liquid transforms into a low density liquid. The presence of a widom line thus confirms previous experiments as it implies the existence of the two liquid states of water.