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Static electricity usually conjures up images of Van de Graaff generators, crazy hair, sticking balloons to walls and the odd shock from an inappropriate clothing choice.
But when Classic Kit columnist Andrea Sella happened to mention a couple of months ago that the cause of static charging is still far from understood, my interest was piqued.
I had assumed from schooldays that it was all sorted out – you rub stuff and it gets charged. But when you think about it, what’s actually causing that charge buildup? Is it really electrons? Surely the work function – the energy required to displace an electron from the surface – of those materials is far higher than simply placing them in contact with another material? What about ions? Or both? Or even bits of the materials themselves transferring over – as I found out researching my latest news piece?
So what’s really going on? The short answer is we really don’t know. That came across talking to Dan Lacks at Case Western Reserve University, US. Lacks told me that he’d originally got into looking at tribocharging when he was approached by a company with a project. ‘I thought it would be easy – I’d just read in the literature how it works and be able to simply solve their problem.’
It turns out to be significantly more complex, and seven years later Lacks is still pondering the issue. In a recent paper of his own, Lacks has shown that touching a rubber balloon to a Teflon surface charges it oppositely depending on whether it’s inflated or deflated, so straining a material changes how it charges.
Not only that, with the advent of modern microscopy techniques, it’s now possible to see what’s happening to charged surfaces at the nanoscale. Last year, Bartosz Grzybowski from Northwestern University, US, showed that – rather than one surface charging positively and the other negatively when 2 materials are rubbed together – both surfaces are covered with tiny mosaic patches of positive and negative charge, and a tiny imbalance of one over the other is responsible for the overall charge.
When you combine that result with his latest work on how nanoscale fragments of the materials are transferred between surfaces on contact, taking their charge with them, it becomes easier to see how material transfer can flip the polarity of the charge on two materials.
But it gets even more interesting when you start to think how that material transfer happens. Grzybowski says that it involves ripping polymer chains off the surfaces, which involves breaking covalent bonds. The same happens when you deform polymers – some of the bonds break and, according to Grzybowski, this produces radicals. If you have the polymers under water when you deform them, then you can produce hydrogen peroxide or stimulate other radical chemistry processes.
To demonstrate how effective the process is, Grzybowski’s team injected a solution of a boronate protected umbelliferone into the sole cavity of some Nike Air trainers. Walking around in the trainers produced enough radicals and H2O2 to cleave the boronate group and release fluorescent umbelliferone.
I’m not sure the people at Nike will be taking it up as a marketing gimmick (especially since you need a UV lamp to see the fluorescence), but it certainly shows that the charge and electronic behaviour of polymers is mind-bogglingly complex and a potential source of some really interesting chemistry – harnessing polymers as a convenient source of mechanically produced radicals could have huge potential when you consider how many industrial and academic processes involve radical pathways.