June 5, 2023
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Scientists solve an 80-year-old physics mystery

The study showed that charge mosaics are a direct consequence of ESD.

Contact electricity (CE) was mankind’s first and only source of electricity until about the 18th century, but its true nature remains a mystery. Today, it is considered a critical component of technologies such as laser printers, LCD manufacturing processes, electrostatic painting, separation of plastics for recycling and many others, as well as a major industrial hazard (damages to electronic systems, explosions in coal mines, fires in chemical plants) due to electrostatic discharges ( ESD) accompanying the CE. A 2008 study published in Nature found that in a vacuum, the ESDs of a simple adhesive tape are so strong that they produce enough X-rays to take an X-ray image of a finger.

For a long time, it was believed that two contact/slide materials are charged in opposite and uniform directions. However, after CE, it was discovered that each of the separated surfaces carries both (+) and (-) charges. The formation of so-called charge mosaics was attributed to the non-reproducibility of the experiment, the inherent inhomogeneities of the materials in contact, or the general “stochastic nature” of CE.

Professor Bartosz A. Grzybowski

Professor Bartosz A. Grzybowski. Credit: UNIST

A research team, led by Professor Bartosz A. Grzybowski (Department of Chemistry) from the Center for Soft and Living Matter, under the Institute of Basic Science (IBS) at the Ulsan National Institute of Science and Technology (UNIST) investigated the possible sources of free mosaics for over a decade. The study is expected to help control potentially harmful electrostatic discharges and was recently published in the journal

Charge Mosaics on Contact Charged Dielectrics

Figure 1. Charge mosaics on contact-charged dielectrics. (a) In a conventional view, two electrically neutral materials (grey) are brought into contact and then separated charge uniformly (lower left), one positive (red) and one negative (blue). In an alternative scenario (lower right), each surface develops a highly non-uniform ‘charge mosaic’ with neighboring domains of opposite charge polarities. (b) Collage of charge mosaics reported in the literature (the years and scale bars are indicated). Credit: UNIST

In the paper published recently in Nature Physics, the group of Professor Grzybowski shows that charge mosaics are a direct consequence of ESD. The experiments demonstrate that between delaminating materials the sequences of “sparks” are created and they are responsible for forming the (+/-) charge distributions that are symmetrical on both materials.

“You might think that a discharge can only bring charges to zero, but it actually can locally invert them. It is connected with the fact that it is much easier to ignite the ‘spark’ than to extinguish it,” says Dr. Yaroslav Sobolev, the lead author of the paper. “Even when the charges are reduced to zero, the spark keeps going powered by the field of adjacent regions untouched by this spark.”

The proposed theory explains why charge mosaics were seen on many different materials, including sheets of paper, rubbing balloons, steel balls rolling on Teflon surfaces, or polymers detached from the same or other polymers. It also hints at the origin of the crackling noise when you peel off a sticky tape – it might be a manifestation of the plasma discharges plucking the tape like a guitar string. Presented research should help control the potentially harmful electrostatic discharges and bring us closer to a true understanding of the nature of contact electrification, noted the research team.

References: “Charge mosaics on contact-electrified dielectrics result from polarity-inverting discharges” by Yaroslav I. Sobolev, Witold Adamkiewicz, Marta Siek and Bartosz A. Grzybowski, 8 September 2022, Nature Physics.
DOI: 10.1038/s41567-022-01714-9

“Correlation between nanosecond X-ray flashes and stick-slip friction in peeling tape” by Carlos G. Camara, Juan V. Escobar, Jonathan R. Hird and Seth J. Putterman, 23 October 2008, Nature
DOI: 10.1038/nature07378

“The mosaic of surface charge in contact electrification” by H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh and B. A. Grzybowski, 23 June 2011, Science.
DOI: 10.1126/science.1201512

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