Saturday, June 25, 2011

Contact!

Sometimes it seems that the oldest problems in science never seem to get solved.  Contact electrification, the transfer of charge by contact between surfaces, was one of the first subjects of science, investigated by Thales of Miletus two and a half millenia ago, and its one of the first sorts of things I remember playing around with in high school physics.  John Timmur, of Ars Technica, has an article on a new Science paper (no reference or link!) in which the process is investigated using Kelvin atomic force microscopy.  The authors found a few surprising results.

But it wasn’t until last year that some of the authors of the new paper published a surprising result: contact electrification (as this phenomenon is known among its technically oriented fans) can occur between two sheets of the same substance, even when they’re simply allowed to lie flat against each other. “According to the conventional view of contact electrification,” they note, “this should not happen since the chemical potentials of the two surfaces/materials are identical and there is apparently no thermodynamic force to drive charge transfer.”

One possible explanation for this is that a material’s surface, instead of being uniform from the static perspective, is a mosaic of charge-donating and charge-receiving areas. To find out, they performed contact electrification using insulators (polycarbonate and other polymers), a semiconductor (silicon), and a conductor (aluminum). The charged surfaces were then scanned at very high resolution using Kelvin force microscopy, a variant of atomic force microscopy that is able to read the amount of charge in a surface.


The Kelvin force microscopy scans showed that the resulting surfaces were mosaics, with areas of positive and negative charges on the order of a micrometer or less across.  All materials they tested, no matter what overall charge they had picked up, showed this mosaic pattern....

So, what causes these charges to build up? It’s not, apparently, the transfer of electrons between the surfaces. Detailed spectroscopy of one of the polymers (PDMS) suggests that chemical reactions may be involved, as many oxidized derivatives of the polymer were detected. In addition, there is evidence that some material is transferred from one surface to another. Using separate pieces of fluorine- and silicon-containing polymers allowed the authors to show that signals consistent with the presence of fluorine were detected in the silicon sample after contact.


It seems plausible that these results are relevant to the classic contact electrification processes that give rise to lightning in thunderstoms and volcanos - and give rise to the Earth's planet wide electric field, about 150 volts per meter near the surface.