I recently reported on a fascinating study on copper in which engineer Parag Banerjee of Washington University in St Louis, USA, and colleagues took a sideways look at the metal using Raman spectroscopy. Copper, like zinc and iron, is an intriguing material in many ways. Whereas most metals simply find themselves coated with a thin metal oxide layer when they are heated, this trio sprout tiny little metal hairs, nanowires, if you will. Banerjee and his team had read the literature on sprouting copper and noticed that most investigators took an aerial view, hence their orthogonal study. They hoped that their novel approach would reveal details of the underlying mechanism and why these nanowires emerge from the bulk rather than a crusty oxide layer forming.
It turns out that ionic transport is involved. As the metal is heated, a relatively thick oxide film forms. However, this film is comprised of two different copper oxides [copper(II) oxide and copper(I) oxide], so revealed the Raman spectra. As these oxides crystallize on the surface of the heated bulk metal, they form with narrow, vertical columns of grain running through them. In between these columns copper ions can traverse the grain boundaries and emerge through the layer as the nanoscopic wires. Quite a neat trick one has to say, but how to visualize it in everyday terms?
As I was writing the article, I was reminded of my children, both in their late teens now, who both, when they were much smaller, enjoyed messing around with "Play-Doh". In case you are not familiar, this is a brightly colored modeling clay material with an evocative and disturbingly appetizing, aroma, familiar to all parents of a certain age. I remember that my children had various moulds, knives and spikes to shape and tease the modeling clay; all consigned to the loft space until any grandchildren come along. There was even a hand-cranked device for extruding spaghetti-like strands of Play-Doh hair for laying on a model head. I also vaguely recall seeing in a toy shop catalog a model head full of holes through which strands might be extruded directly. Now, this image was not dissimilar to the mechanism being described by Banerjee, but obviously on a completely different scale and with non-metallic ingredients.
However, as I was modeling a metaphor for the article it occurred to me that Play-Doh is perhaps not universally recognizable after all and that a slight variation on the image was needed for a wider audience, particularly one with a technical bent and a wide age range. So, having discarded the modeling clay, I crafted a metaphor in which the sprouting copper nanowires were strands of ground beef extruded from a mincing machine. Why I didn't just think of the spaghetti maker at the time I do not know. Needless, to say although the Play-Doh metaphor was childish, the virtual schematic of the ground beef was perhaps even more inappropriate for vegetarians and anyone who does not eat beef for whatever reason. The beefy metaphor did not make the final cut as published in my column on SpectroscopyNow, you will presumably be pleased to learn. Indeed, no metaphor was used. I simply described the observations and the description of copper nanowires emerging between grain boundaries. I reasoned that if you are bright enough to understand grain boundaries you probably do not need mixed metaphors with meat products and modeling clay.
Of course, as with much fundamental materials science, there are putative technological applications on the horizon that appear as a result of the clearer understanding of how these copper nanowires grow and how they might be exploited in solar-energy conversion, semiconductor sandwiches and water-splitting. I do not now intend to slice all my scientific metaphors, they have their place particularly in popular science communication, although I must confess that sandwich I just mentioned does not sound particularly appetizing.
David Bradley blogs at http://www.sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".