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Light Controls Surface-Charging

Electrostatics: Method could reveal mechanism of ubiquitous phenomenon behind static cling, copy machines

by Amanda Yarnell
September 17, 2010 | A version of this story appeared in Volume 88, Issue 39

Polymers with spiropyran groups reversibly form zwitterionic merocyanines when exposed to UV light, changing the polymer’s contact electrification properties in the process.
Polymers with spiropyran groups reversibly form zwitterionic merocyanines when exposed to UV light, changing the polymer’s contact electrification properties in the process.

A strategy using light to control contact electrification paves the way for better understanding of the phenomenon, which underlies processes such as static cling, laser printing, and the production of cleaner-burning coal.

Contact electrification—also known as triboelectric charging—is the electric charge that builds up on surfaces when they rub together and then separate. This built-up charge is what makes a balloon stick to a wall after it has been rubbed on your hair and what makes your socks stick to your pants when they come out of the dryer.

Despite the phenomenon's ubiquity, "scientific understanding of triboelectric charging is extremely poor," says Daniel J. Lacks of Case Western Reserve University. "Even the most basic questions are not understood. For example, it is not clear whether the charge that is transferred is carried by electrons or ions."

The study of triboelectric charging has been hampered by scientists' inability to reproduce the effect quantitatively, explains Samuel W. Thomas III of Tufts University. The method he and postdoc Simone Friedle have designed should fill that gap, he says. The researchers use light to reversibly alter the chemical composition of a surface such that the sign of the electric charge left on a steel ball changes after the ball rubs with the modified surface (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201003985).

"This development of a method to change triboelectric charging so reproducibly may be the key to further experiments that definitively determine what type of species is being transferred in the charging process," Lacks says.

Thomas also thinks the strategy might someday lead to electrostatic-driven actuators or ways to prevent potentially damaging static discharge in electronics manufacturing or fueling stations.

In the experiments, a steel ball rolls in a circular pattern on a film made of a spiropyran-based photoreactive polymer. As the ball rolls, it periodically passes over an electrode that measures the charge the ball builds up in real time.

The researchers find that the spiropyran film reversibly changes contact electrification behavior when exposed to ultraviolet light, which converts the film's spiropyran groups to less hydrophobic merocyanine groups. The ball charges positively when rolling on the spiropyran film, but negatively when rolling on the merocyanine film. Visible light reverses the effect, Thomas says.

The results suggest that a surface's hydrophobicity may play a role in contact electrification behavior, Thomas adds. His group is now designing other organic photochromic films with switchable redox potentials, electronic states, and hydrophobicities in hopes of sorting out which charged species migrate during contact electrification.



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