In a finding that has been met with surprise and some
controversy in the scientific community, researchers at MIT and elsewhere have
discovered a basic property that governs the way water and many other liquids
behave as their temperature changes.
Liquids have long been known to exhibit a rapid change
in properties near a point called the glass transition temperature, where the
viscosity of the liquid becomes very large. But MIT professor Sow-Hsin Chen and
his co-researchers have found a different transition point at a temperature
about 20% to 30% higher, which they call the dynamic crossover temperature.
This temperature may be at least as important as the glass transition
temperature, and the viscosity at the dynamic crossover temperature seems to
have a universal value for a large class of liquids (called glass-forming
liquids) that includes such familiar substances as water, ammonia and benzene.
At this new transition temperature, “all the transport
properties of the liquid state change drastically,” Chen says. “Nobody realized
this universal property of liquids before.” The work, carried out by physics
professor Francesco Mallamace of the Univ. of Messina, Italy (who is a research
affiliate at MIT) and four of his students from Messina, along with Chen, an
MIT professor emeritus of nuclear science and engineering, and Eugene Stanley,
a physics professor at Boston Univ., was published in the Proceedings of the National Academy of Sciences.
This is very basic research and Chen says it is too
early to predict what practical applications this knowledge could produce. “We
can only speculate,” he says, because “this is so new that real practical
applications haven’t really surfaced.” But he points out that one of the most
widely used building materials in the world, concrete, flows as a liquid-like
cement paste during construction, and a better ability to understand its
process of transition to solid form might be significant for improving its
durability or other characteristics.
Controversial finding
The team had previously published their findings about the new transition temperature
in water, but the new work extends this to the whole class of liquids. While
the findings remain somewhat controversial, Chen says that last month an
international symposium devoted to the study of this phenomenon was held in Florence, Italy
involving about 50 scientists from various nations.
Benjamin Widom, an emeritus professor of chemistry at
Cornell Univ., says that the researchers’ demonstration of the universality of
this crossover phenomenon and the fact that the liquids studied all show
roughly the same level of viscosity at their crossover point “is striking,” and
adds that “These observations are certain to arouse much interest among those
who work in the field, and perhaps even controversy because they contradict
long-accepted ideas.”
Liquids become much more viscous as they approach
their freezing temperature. But the exact progression of this transition is
difficult to measure, so the details are still poorly understood. The new
research draws on published studies detailing the behavior of 84 different
liquids, and the researchers found that a fresh analysis of the data, along
with their own experimental work on water, shows a previously unrecognized
universal property they all share in terms of how their viscosity and other
characteristics change with temperature.
“Measuring viscosity is a very tedious process,” Chen
says, and measuring how it changes over tiny increments of temperature is even
more difficult. But Chen and his colleagues found that they were able to measure
relaxation time of water—which is directly proportional to its viscosity—using
a state-of-the-art instrument at the National Institute of Standards and
Technology in Washington
that shoots neutrons at the material. “We discovered we can measure relaxation
time very effectively with this instrument,” Chen said, and they have been
carrying out such measurements over the last several years.
The research was supported in U.S. by grants from the U.S. Department of
Energy and the National Science Foundation and in Italy from the Research Programs of
National Interest.
