Static electricity mystery solved
Image: Changes in microstructure, such as void and fibrils created by straining a polymer sheet, appear to control how the material charges through friction (scale bar: 50 micron).
For centuries, scientists have tried to understand triboelectric charging, commonly known as static electricity.
But new results from researchers at Case Western Reserve University, US, indicate that microstructural changes in a material can control how it becomes electrically charged through friction.
“Our idea was that a strain on the materials was causing a higher propensity for the materials to become charged,” highlights Professor Dan Lacks from the Department of Chemical and Biomolecular Engineering at the university.
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.
The 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 tended to carry a negative charge and the strained film a positive charge. The finding was not consistent 100 percent of the time, but statistically significant.
In contrast, unstrained films rubbed together and strained films rubbed together appeared to charge at random.
Collaborators at Bilkent University, in Ankara, Turkey, went on to use X-ray diffraction and Raman spectroscopy to analyse samples of strained and unstrained films and found the samples were almost identical at the atomic level .
However, they also observed that the strained film contained voids; some of these holes and fractures were detected with the naked eye while others could only be observed under a scanning electron microscope.
The researchers created molecular simulations of strained materials on a computer, which showed the birth of the voids but no other significant changes.
These results also indicated that the change in microstructure would be the likely cause of the systematic charge transfer.
“We think the void regions and the fibrils we see around them when we strain the polymer have different bonding and thus charge differently,” explains Lacks.
"No significant changes in the molecular-level structure of the polymer are observed by XRD and micro-Raman spectroscopy after deformation," he adds. "However, the strained surfaces were found to exhibit void and craze formation spanning the nano- to micrometer length scales by molecular dynamics simulations, SEM, UV-vis spectroscopy, and naked-eye observations."
According to the researcher, this experiment focused on one material, but strain may affect all materials, so he and colleagues are now looking at the effects of strain on granular materials as well as other polymers, including polystyrene peanuts and plastic bags.
Research is published in Physical Review Materials.