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Until very recently, computer simulations of so-called model liquids were the only method capable of probing molecular motion at this level of detail. In a model liquid, the individual atoms or molecules interact via specified forces and move according to Newton’s second law of motion. Over the past few years, such simulations have provided many intriguing predictions concerning the co-operative nature of molecular motion in glass-forming liquids. One of the most striking is that a population of last molecules emerges amidst the increasingly sluggish motion in the liquid. These fast molecules move co-operatively in string-like paths, and cluster into transient, fractal structures that grow rapidly in size as the glass transition is approached.

These predictions have now been confirmed by Eric Weeks and co-workers at Harvard University, the Universilv of Pennsylvania and Edinburgh University. The research team made the breakthrough studying suspensions of colloids that form glasses when they become very dense.

Colloids are large particles, typically between several nanometres and a few microns in size, and are found in paints, inks, cosmetics and other everyday products. Scientists also think of them as model liquids because they obey the laws of statistical mechanics. Colloids that act essentially as hard spheres and interact only when they touch are perhaps the simplest example of a glass-forming system, Because of their large size, which is roughly the wavelength of visible light, colloids can be seen through an optical microscope.

Weeks and co-workers used a confocal microscope - which can focus to different depths in the suspension - a video camera and image-analysis software to record the positions of individual colloidal particles in 3-D over many hours. In this way, the researchers found that roughly 5% of the particles moved together in groups or “clusters” at any given time, while the rest of the liquid remained relatively static. As predicted by simulations of several model glass-formers, a wide range of cluster sites was observed, and the typical cluster site grew rapidly as the glass transition was approached.

Willem Kegel and Alfons van Blaaderen of the University of Utrecht in the Netherlands have also observed correlated particle motion as well as regions of high and low mobility in similar experiments with colloidal suspensions. However, their experiments were restricted to observing motion within 2-D slices of the sample.

Both studies demonstrate the increasing significance of co-operativity in enabling particle motion as the colloids become denser. Like in the train-station analogy, as the particles become crowded, their motion becomes increasingly difficult. An increasing number of particles must move together in order to move at all.

The pioneering work of Weeks and co-workers establishes confocal optical microscopy as a powerful tool with which to probe the dynamics of fluids composed of large numbers of micron-sized particles. The researchers are quick to acknowledge the role of simulation in inspiring and guiding their experiments. Indeed, computer simulations played a unique role in identifying the most relevant quantities to be measured. They have also lead to a statistical-mechanical framework within which to describe correlated particle motion.

 The partnership between experiment and simulations is ideally suited to addressing the outstanding questions central to the glass transition. How does co-operative motion arise in the first place. How is the particle motion related, to the underlying liquid structure? And how is co-operativity related to the overall slowing-down of relaxation in the liquid?