Static Electricity Re-visited

Mother and child enjoying electrification

Mother and child enjoying electrification

An interesting article over at Ars Technica claims that a recent paper in Science means that everything you thought you knew about static electricity is wrong. I would put it differently, and rather less dramatically. The paper reports an experimental investigation into the process of ‘contact electrification’ in dielectrics (insulators in everyday parlance). This is the process which takes place when you rub a balloon on a jumper. Their experiments show that when two uniform flat  insulating surfaces touch, the physical and chemical behaviour is more complicated than had been envisaged and the concept of a simple ‘transfer of charge’ is really overly simplistic. However, for anyone who has studied electrostatic phenomena it hardly comes as a surprise to learn that ‘its a bit more complicated’ than the simplistic story of the triboelectric series.

What the paper says:

  • When two uniform flat dielectric surfaces touch, the physical and chemical behaviour is more complicated than had been envisaged. Please note that the word ‘touch‘ which we all understand colloquially is actually not properly scientifically defined if we look at the nanoscale. Broadly speaking it means the surfaces get ‘close enough that the electric forces between atoms in the surface become very strong’.
  • If two different dielectrics ‘touch’ there can be a net transfer of electric charge from one surface to the other. This is as envisaged in the simple model. The paper shows that even in this case electrical charge goes both ways leading to a mosaic pattern on both surfaces of regions of positive and negative charge.
  • There can also be transfer of ions as well as electrons. If the paper were written by physicists, that would be the end of it. But because the effects involve a transfer of electrons AND ions the authors now say the reactions are chemical. I think this is a case where some chemists are trying to barge in on physics territory!
  • The surface charge pattern decays with a time constant of around 1000 seconds (15 minutes) but not due to surface charge transport, rather due to interactions with ions in the air. Again, anyone who has stuck a balloon to the wall could testify to the lifetime of the electrification effect.
  • Even two nominally identical surfaces when brought into ‘contact’ and then separated will charge in a ‘mosaic’ pattern, with charge going both ways. This is because on the nano-scale, the surfaces are not identical. On average there is approximately zero charge transfer, but this made up of lots of small transfers in both directions.
PDMS

Two surface maps showing a nano-scale voltage measurement across the surface on a insulator called polydimethylsiloxane (PDMS). The top map shows an area 4.5 micrometres square before touching another piece of PDMS. The bottom map shows the same are after contact electrification. Notice the surface has become negatively charged in some areas and positively charged in others.

To me main point is that the details of the charge transfer processes between two different surfaces are more complex than had previously been envisaged. In particular, the charge transfer depends on the local situation on each surface, where local is referring to areas [a few hundred atoms] x [a few hundred atoms]. The researchers are not saying the previous view was wrong, merely that it was oversimplified. But as I mentioned above, for anyone who has studied or thought about these phenomena it can hardly come as a surprise to learn that ‘its a bit more complicated’ than the simplistic story.

What is nice about the work is that it is mainly phenomenological – it just says ‘we did these experiments and saw this’. And it is great to revisit ‘the familiar’ with new techniques and see the complex and the mysterious in the process taking place around us every day. It helps to keep us all on our toes and make sure the stories we tell our children are truly based on best science, and are not simply fictions that sound scientific.

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