Manufacturers who design new materials often struggle to understand viscous liquids at a molecular scale. Many substances including polymers and biological materials change upon cooling from a watery state at elevated temperatures to a tar-like consistency at intermediate temperatures, then become a solid "glass" similar to hard candy at lower temperatures. Scientists have long sought a molecular-level description of this theoretically mysterious, yet common, "glass transition" process as an alternative to expensive and time-consuming trial-and-error material discovery methods. Such a description might permit the better design of plastics and containers that could lengthen the shelf life of food and drugs.
A fundamental question is why many materials behave differently when temperature changes. In some "fragile" glass-forming materials, a modest variation in temperature can make the material change from highly fluid to extremely viscous, while in "strong" fluids this change in viscosity is much more gradual. This effect influences how long a manufacturer has to work with a material as it cools. "For decades, material scientists have heavily relied on empirical rules of thumb to characterize these materials," says NIST theoretician Jack Douglas. "But if you want to design a material that does precisely what you want, you need a molecular understanding of the underlying physical processes involved."
According to Douglas, the increasingly viscous nature of glass-forming liquids is related to molecules that move together in long strings around other atoms that are almost frozen in their motion. The growth of these snake-like structures leads to an increase in the viscosity of the liquid: the lower the temperature, the longer the chains, and the more viscous the fluid. The team found that the rate at which these spontaneously organizing snake-like strings grow in size as the material cools is quantitatively related mathematically to the fluid fragi
|Contact: Chad Boutin|
National Institute of Standards and Technology (NIST)