A recent press release, describing a paper by Princeton theoretical physicists Rechtsman, Stillinger,and Torquato, begins with the stirring words “It has been 20 years since the futurist Eric Drexler daringly predicted a new world where miniaturized robots would build things one molecule at a time. The world of nanotechnology that Drexler envisioned is beginning to come to pass….” The mention of Drexler has ensured that the release got a mention on the Foresight Institute’s blog, Nanodot, but Christine Peterson disarmingly appeals for help in understanding what on earth the release is talking about. Fair enough, in my view; whatever one thinks of the Drexler reference, this is one of the worst written press releases I’ve seen for some time.
A look at the original paper, in Physical Review Letters, (abstract here, preprint here, subscription required for published paper) gives us more of a clue. The backstory here is the fact that collections of spherical particles in the size range of tens to hundreds of nanometers can (if they’re all the same size) spontaneously self-assemble to form ordered arrays, often called “colloidal crystals”. The gem-stone opal is a natural example of this phenomenon; it’s formed from naturally occurring silica nanoparticles, and its iridescent colours are a result of light diffraction from the crystals. It is these striking optical properties that have raised research interest in synthetic analogues; for some sets of parameters it’s predicted that these materials might have an “optical bandgap” – a range of wavelengths of light that can’t get through the crystal in any direction. This would be useful, for example, in making highly efficient solid state lasers. The problem is that most systems of simple spheres form close-packed crystal structures – of the kind you get when stacking oranges. But it would be useful if one could make colloidal crystals with different structures, such as the diamond structure, which have more interesting potential optical properties. In principle one might be able to do this by tinkering with the interaction potentials between the particles. Close packed structures occur because the particles simply attract each other more and more until they touch, at which point they resist further compression. What this paper shows is that you can design potentials to produce the crystal structure you want – perhaps you need the particles to attract to each other up to a certain distance, then softly repel until they get a bit closer, and then start to attract again until they touch. This is an elegant piece of statistical mechanics. Of course, having designed the potential theoretically you still need to design a system that in practise has these properties. One can imagine how to do this in principle, perhaps by having colloids that combine a tunable surface charge with a soft polymer coating, but such a demonstration needs a lot of further experimental work.
Is this really “turning a central concept of nanotechnology on its head” ? Of course not. It’s a nice step forward in theoretical methods, but it’s absolutely in the mainstream of a well established research direction for obtaining interesting ordered structures by colloidal self-assembly. And as for the next sentence – “If the theory bears out – and it is in its infancy — it could have radical implications not just for industries like telecommunications and computers but also for our understanding of the nature of life” – I can only hope the authors are cringing as much as they should be at what their publicists have put out for them.
Updated with link to preprint Tuesday 20.50.