We Finally Know How Friction Causes Static Electricity (Physics)

A new model shows how rubbing two objects together creates static electricity, the answer to a mystery that has confounded scientists for more than 2,500 years.

The model that shows that rubbing two objects together produces static electricity, also known as triboelectricity, by bending the tiny protrusions on the surface of materials. This new understanding could have important implications for existing electrostatic applications, such as energy harvesting and printing, as well as for avoiding potential dangers, such as fires started by sparks from static electricity.

Greek philosopher Thales of Miletus first reported friction-induced static electricity in 600 B.C.E. After rubbing amber with fur, he noticed the fur attracted dust.

“Since then, it has become clear that rubbing induces static charging in all insulators — not just fur,” says Laurence Marks, a professor of materials science and engineering in the McCormick School of Engineering at Northwestern University, who led the study. “However, this is more or less where the scientific consensus ended.”

At the nanoscale, all materials have rough surfaces with countless tiny protrusions. When two materials come into contact and rub against one another, these protrusions bend and deform.

Marks’s team found that these deformations give rise to voltages that ultimately cause static charging. This phenomenon is called the “flexoelectric effect,” which occurs when the separation of charge in an insulator arises from deformations such as bending.

Using a simple model, the researchers showed that voltages arising from the bending protrusions during rubbing are, indeed, large enough to cause static electricity. This work explains a number of experimental observations, such as why charges are produced even when we rub two pieces of the same material together and predicts experimentally measured charges with remarkable accuracy.

“Our finding suggests that triboelectricity, flexoelectricity, and friction are inextricably linked,” Marks says. “This provides much insight into tailoring triboelectric performance for current applications and expanding functionality to new technologies.”

“This is a great example of how fundamental research can explain everyday phenomena which hadn’t been understood previously, and of how research in one area — in this case friction and wear — can lead to unexpected advances in another area,” says Andrew Wells, a program director at the National Science Foundation, which funded the research.

References: C. A. Mizzi, A. Y. W. Lin and L. D. Marks, “Does Flexoelectricity Drive Triboelectricity?”, Physical Reviews Letters, pp. 1-33..

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