So, how does cling wrap cling? Is it just sticky, or does it have something to do with static electricity?
The answer is yes and no. Yes it really is sticky, and no, it’s mostly not static electricity – but also yes it is a little bit.
First, the stickiness. Plastic wrap is usually made from something like polyvinyl chloride (PVC) or low density polyethylene (LDPE), to which they often add LLDPE as well (that’d be linear low density polyethylene).
These substances are naturally clingy without static electricity (LDPE less so, but the LLDPE helps). Static electricity is, after all, an electrical charge that’s usually built up by rubbing two non-conducting materials, or insulators, together. Like rubbing your feet on the carpet, or rubbing a balloon against your hair.
The usual explanation for what’s happening here is that by rubbing the materials together you’re transferring electrons from one to the other – so one piece is positively charged, the other negative.
(Incidentally, this is where electricity gets its name: the ancient Greeks discovered they could charge rods of amber by rubbing them with fur, and so in 1600 when the English scientist William Gilbert did his own experiments he named it after the Greek word for amber, ήλεκτρον, or electron.)
Of course, the thing with plastic wrap is that it can also stick to metal or damp surfaces, which would be expected to conduct away any accumulated static electric charge.
But hang on! If you play with plastic wrap you’ll notice that it doesn’t just cling to surfaces, it also attracts things towards it, just like anything else charged with static electricity. And this is where things get complicated…
Because our plastic wrap has picked up its charge by being pulled off a roll of other plastic wrap. So if the usual explanation for static electricity is right, how was charge transferred between two identical substances? Why would one piece be positive and the other negative?
Thankfully, a recently published paper may give us the answer. Researchers from Northwestern University in Illinois US used Kelvin force microscopy, which is able to measure the amount of charge on a surface, to examine two pieces of similar material that been brought together (Baytekin HT, Patashinski AZ, Branicki M, Baytekin B, Soh S & Grzybowski BA, “The mosaic of surface charge in contact electrification”, Science DOI: 10.1126/science.1201512, published online 23 June 2011).
What they found was that you didn’t get a uniform arrangement where one piece was all positive and the other was all negative. Instead, each surface was covered in a mosaic of both positive and negative charges, in tiny areas less than a micrometre across.
Overall there’s a slight imbalance, so the material behaves as if it’s got a net charge. But the charge density in each of these microscopic regions is much, much higher than you’d expect.
And these charges seem to be caused by chemical reactions taking place on the surface. The materials used in the experiment were similar, but not identical: one contained fluorine, one contained silicon, and when they tested them afterwards, they found that some of the material had been transferred.
So that’s what seems to be going on with our plastic wrap! There is static electricity, but it’s more complicated than the one-way transfer we’re all used to.
And it also shows there’s always something new to learn, even about phenomena we’ve thought we understood for hundreds of years.