Run your fingers over a smooth polystyrene surface and you would be convinced that it is hard and solid. James Forrest has news for you — that surface is still liquid and continues to flow even at the freezing point of the material.
The University of Waterloo physicist recently co-authored a paper in Science that suggests this counter-intuitive observation can be found in any material that forms a glass, from simple compounds like glycerol to complex polymers. “Glasses are the most common solids,” Forrest says. “It’s a much more common state of matter than crystalline. Everything can form a glass and there are materials that will never crystallize.”
Although the transition point where glasses form would appear to be an all-or-nothing proposition, Forrest says their results show otherwise. “What we’ve learned is that in some materials — but we think all materials — even at temperatures well below the transition where the materials acts like a glassy elastic solid, the surface doesn’t,” he explains. “The surface refuses to vitrify and it just continues to flow.”
The surface layer in question may be just a few nanometres thick, but when the researchers cut tiny steps in that layer on a seemingly solid piece of polystyrene, it flattened out in comparatively short order. At a temperature 100 degrees cooler than the glass transition point, such deformations healed in three weeks.
This tantalizing finding leaves Forrest to weigh the distinction between the macroscopic world that is familiar to us and this nanometre-scale environment where matter behaves somewhat differently. “We pick up an iPhone and rely on the fact that its manufacturer could successfully conduct lithography in surfaces at the nanometre scale, even though our intuition is wrong at that scale,” he says.
And while this work sheds new light in other areas, such as the behaviour of thin films, Forrest still has much to consider about simple polystyrene. “We don’t have an answer to the question: does the surface ever freeze? I have data to suggest that it never does.”