Why rubbing a balloon on your hair makes it stick

Tiny changes in the surface of a blown up balloon causes it to charge more when rubbed against our hair and stick to other surfaces, say scientists who have decoded why some materials acquire static charge better.

For centuries, scientists have tried to understand triboelectric charging, commonly known as static electricity.
Triboelectric charging causes toner from a photocopier or laser printer to stick to paper, and likely facilitated the formation of planets from space dust and the origin of life on earth, researchers said.

However, the charges can also be destructive, sparking deadly explosions of coal dust in mines and of sugar and flour dust at food-processing plants.

New research led by Case Western Reserve University in the US shows that tiny holes and cracks in a material - changes in the microstructure - can control how the material becomes electrically charged through friction.

The research is a step toward understanding and, ultimately, managing the charging process for specific uses and to increase safety, the researchers said.

"Electrostatic charging can be seen everywhere, but we noticed some cases where materials appeared to charge more - like a balloon rubbed on your head, or packing peanuts sticking to your arm when you reach into a package," said Dan Lacks, from Case Western Reserve University.

"Our idea was that a strain on the materials was causing a higher propensity for the materials to become charged," Lacks said.

"After blowing polystyrene to create the expanded polystyrene that comprises the peanut, the material maintains this distinct charging behaviour indefinitely," he said.

To test the theory that strain affects charging, the researchers stretched a film of polytetrafluoroethlyne (PTFE) and rubbed it against a film of unstrained PTFE.

"Triboelectric charging experiments are generally known for their inconsistent results," said Andrew Wang, a PhD student at Case Western Reserve.

"What was surprising to me, initially, was the consistency of the unstrained versus strained charging results," said Wang, who led the study published in the journal Physical Review Materials.

Researchers repeatedly found a systematic charge transfer in one direction, as if the materials were made of two different chemical compositions.

After rubbing, unstrained films clearly tended to carry a negative charge and the strained film a positive charge. The finding was not consistent 100 per cent of the time, but statistically significant.

In contrast, unstrained films rubbed together and strained films rubbed together appeared to charge at random.
Researchers used X-ray diffraction and Raman spectroscopy to analyse samples of strained and unstrained films and found at the atomic level, they looked nearly the same.

The only detectable difference in the strained film from the unstrained film was the presence of voids in the material - holes and fractures created by stretching, which changed the microstructure.

"We think the void regions and the fibrils we see around them when we strain the polymer have different bonding and thus charge differently," Lacks said.