Credit: HARVARD PHOTO SERVICES

Glass is traditionally made from liquids — composed of molecules moving freely about — that become solid without crystallizing. But glass can also be made from other substances that end up more or less 'glass-like'. David Weitz's physics lab at Harvard University in Cambridge, Massachusetts, found that soft colloidal particles — bits of solid suspended in a liquid — can be made to form glass with remarkably similar properties to more traditional kinds (see page 83).

Most attempts at using colloids to make glass have focused on the more common 'hard' varieties of colloids. But Weitz was intrigued by soft colloids. “We realized their behaviours were very different,” he says. Most colloid particles — especially hard ones — are spherical. But soft colloid particles can become irregularly shaped when they are squeezed together in greater densities. Would this property modify the motion of the particles, changing the way they solidify into a glass?

Weitz did not initially set out to study how these soft, more deformable, colloids might form glass. The lab's movement in that direction came when postdoc Johan Mattsson, who had studied glass formation during his PhD training in Sweden, joined the lab. Meanwhile, Zhibing Hu, a colleague from the University of North Texas in Denton, had been creating and characterizing a unique class of microgel — soft colloidal particles made by combining two different polymers that allow control of the degree of softness of the resultant particles.

The group started thinking that using these particles, and controlling their softness, might enable them to tease out how this characteristic affects glass formation. “It was a convergence of interests and diverse expertise that led to these results,” says Weitz.

The lab used three colloids of different softness, and mixed them at various concentrations and temperatures. They found that changing the softness of the particles could produce very different behaviours. “When you can heat the particles up, you can shrink them, by taking some water out of them,” says Weitz.

Next they observed how glass-like their mixtures were by using light-scattering techniques and measuring the substances' responses to mechanical force. They then compared measurements in soft-colloid-derived glasses with more standard-type glasses.

They found that the softer, more deformable particles approached the glass state much more gradually than the stiffer, less deformable ones. This richness in behaviour is similar to that of more traditional molecular glasses, which behave differently depending on the type of molecules from which they are formed. It had never, however, been seen in colloidal or particulate glass-forming systems.

Understanding glass formation by soft colloids is of more utility in understanding other materials than in practical applications as a new kind of material. “Most practical systems are made up of what I call soft materials — particulates, emulsions, polymers, suspensions — things that are around us in our everyday life,” says Weitz. “Many of these systems will exist in some kind of glass-like state. The nature of this glass-like state is one of the most important features to make these materials useful.”