For many of us, static electricity is one of the earliest encounters we have with electromagnetism, and it’s a staple of high school physics. Typically, it’s explained as a product of electrons transferred in one direction between unlike substances, like glass and wool, or a balloon and a cotton T-shirt (depending on whether the demo is in a high school class or a kids’ party). Different substances have a tendency to pick up either positive or negative charges, we’re often told, and the process doesn’t transfer a lot of charge, but it’s enough to cause a balloon to stick to the ceiling, or to give someone a shock on a cold, dry day.
Nearly all of that is wrong, according to a paper published in today’s issue of Science. Charges can be transferred between identical materials, all materials behave roughly the same, the charges are the product of chemical reactions, and each surface becomes a patchwork of positive and negative charges, which reach levels a thousand times higher than the surfaces’ average charge.
Where to begin? The authors start about 2,500 years ago, noting that the study of static began with a Greek named Thales of Miletus, who generated it using amber and wool. 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.â€
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. The charges will dissipate over time, and the authors found that this process doesn’t seem to occur by transferring electrons between neighboring areas of different charge—instead of blurring into the surroundings, peaks and valleys of charge remain distinct, but slowly decrease in size. The authors estimate that each one of these areas contains about 500 elementary charges (that’s ±500 electrons), or about one charge for each 10nm2.
The reason that this produces a relatively weak charge isn’t because these peaks and valleys are small; the charge difference between them is on the order of 1,000 times larger than the average charge of the whole material. It’s just that the total area of sites with positive and negative charges are roughly equal (the two are typically within a fraction of a percent of each other). The distribution appears to be completely random, as the authors were able to produce similar patterns with a white noise generator that fluctuated on two length scales: 450nm and 44nm.
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.
