Comment & Analysis
“AN UNLUCKY SCRAP OF SILK WAS ABOUT TO BECOME A SCIENCE EXPERIMENT, RATHER THAN A DRESS”
Helen Czerski on static electricity
Surrounded by lovingly coordinated racks of beautiful fabrics, and hushed discussions about bias cuts and fancy stitching, I felt as though I was about to commit the ultimate fabric shop sin. It wasn’t asking for pure silk that was the problem. It was admitting that some unlucky scrap of fabric was about to be denied the glory of becoming an evening dress, and would instead find a home in a science experiment. This is because I had read that silk is the perfect material with which to charge a plastic comb with static electricity. The assistant raised his eyebrow and left me to it.
The next day we did the demo and the silk had its moment of glory. Except that it didn’t. When rubbed along the comb, my cotton T-shirt did just as well or just as badly, depending on how you look at it. Neither one charged the comb reliably. And so I did some digging to find out why.
At the heart of all these static electricity demos is a simple principle: when two materials are rubbed together, some electric charges hop from one to the other, giving each object a residual electric charge (which we call static electricity). This is the triboelectric effect. It’s most obvious for electrical insulators in dry air, because the charge that is generated on the surface tends to stay put and can’t escape either into the air or the depths of the object.
I notice it most when laundry comes out of the dryer and is stuck together in one big static lump, but that doesn’t happen every time. The majority of my clothes are made from cotton, except for my polyester sports kit. Static is worst are when both are mixed in together. This might be explained by something called the ‘triboelectric series’, which is an ordered list of materials telling you which ones get positively and negatively charged. If you rub together something on the ‘positive’ side (for example, glass) with something on the ‘negative’ side
(for example, polyester), you’re more likely to generate a strong charge. The problem is that the triboelectric series is only approximate – if you do the tests on a different day, you’re quite likely to get a different order. Cotton is generally found around the middle of the triboelectric series and polyester is well into the negative end, setting me up for very clumpy laundry on days where I mix the two fabrics.
The thing that surprises me the most is that even the most recent research papers admit that no one is completely sure why this all happens, especially for a pairing like silk and a plastic comb. If you rub two metals together, everyone agrees that electrons hop across, and then the metal conducts the charge away. But if you rub two electrical insulators together, like silk and plastic, it’s not that simple. The best current explanation seems to be that the friction generated by rubbing them together encourages small charged fragments of molecules (known as mobile ions) to jump the gap. The heat and the pressure between the rough surfaces may even cause chemical reactions which form those ions. But there’s a lot of nuance, which is why it’s hard to predict which way the charges will jump on any given day.
And silk? Silk is close to zero on most versions of the triboelectric series, next to cotton. Perhaps my scrap of shiny fabric would have been better off in a dress. Perhaps one day silk really will swoosh into science demos with some properly electric glamour.