Solve the lingering mystery of the relationship between friction and static electricity

Most people felt the disconcerting effect of rubbing a balloon on the head or the subtle sparkle caused by training feet on the carpet. Although these experiences are common, a detailed understanding of how they have occurred has escaped scientists for over 2500 years.

Now, a team from Northwestern University has developed a new model that shows that the friction of two objects produces static electricity, or triboelectricity, by bending the tiny protrusions on the surface of the materials.

This new understanding could have important implications for existing electrostatic applications, such as energy recovery and printing, as well as to avoid potential hazards, such as fires caused by sparks coming from the environment. 39, static electricity.

The research will be published Thursday, September 12 in the journal Letters of physical examination. The study was conducted by Laurence Marks, professor of materials science and engineering at the Northwestern McCormick School of Engineering. Christopher Mizzi and Alex Lin, PhD students at Marks Lab, were the co-first authors of the article.

The Greek philosopher Thales of Miletus reported for the first time friction-induced static electricity in 600 BC. After rubbing amber with fur, he noticed that fur attracted dust.

"Since then, it has become clear that friction induces a static charge in all insulators – not just in fur," said Marks. "However, it is more or less where the scientific consensus has ended."

At the nanoscale, all materials have rough surfaces with innumerable small protrusions. When two materials come in contact and rub against each other, these protrusions bend and deform.

The Marks team found that these deformations generated tensions that ended up causing a static charge. This phenomenon is called "flexoelectric effect", which occurs when the separation of charges in an insulator results from deformations such as bending.

Using a simple model, the North West team showed that the tensions resulting from the bending protuberances during friction were, in fact, high enough to cause static electricity. This work explains a number of experimental observations, such as the reasons why charges are produced even when two pieces of the same material are rubbed and predicts loads measured experimentally with remarkable accuracy.

"Our discovery suggests that triboelectricity, flexoelectricity and friction are inextricably linked," said Marks. "This provides a lot of information on customizing triboelectric performance for current applications and extending functionality to new technologies."

"This is an excellent example of how basic research can explain everyday phenomena that have not yet been understood, and how research in a field, in this case the friction and wear, can lead to unexpected advances in another area, "said Andrew Wells. , program director at the National Science Foundation (NSF), who funded the research. "NSF is funding research like this in Materials Science and Engineering for new insights that may one day open up new opportunities."