Com­ment & Anal­y­sis

“AN UN­LUCKY SCRAP OF SILK WAS ABOUT TO BE­COME A SCIENCE EX­PER­I­MENT, RATHER THAN A DRESS”

BBC Earth (Asia) - - Contents -

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.

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