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Two-faced Electronic Paper

Janus

Two-faced microscopic beads that rotate through a half turn when an electric field is applied to them could be the key to creating electronic paper, according to Japanese scientists. Takasi Nisisako and his team in the Department of Precision Engineering at the University of Tokyo have developed a new technique that allows them to produce these “Janus” particles much more efficiently and with uniform size.

Other researchers have previously developed two-colored beads of between 30 and 150 micrometers diameter (a micrometer is a thousandth of a millimeter) for experimental electronic paper applications. A thin film of these particles is sandwiched between a control grid and a protective surface. Applying a voltage to specific regions of the layer through the control grid can make clusters of individual particles flip over so that they appear black against a background of un-flipped white particles. Such technology could allow a device to display an image, words or pattern that is retained using no additional power until an erase voltage is applied making the particles flip back to white.

Such electronic paper has not yet entered the mainstream electronic gadgetry market because making the particles all the same size and of consistent quality is difficult. Prototype devices based on these Janus particles cannot yet produce a perfect picture. Nisisako and his team have now solved the quality control problem by side-stepping the standard manufacturing approach and have instead turned to microfluidic technology.

The team built a tiny device that comprises a sliver of glass into which is etched a Y-shaped channel. The researchers then seal this beneath a second layer of glass leaving the ends of the channel open. The two starting ingredients are liquid monomers designed to produce particles that respond to an electric current and have a different color on each of their two faces. The team feeds each ingredient into the arms of the “Y” where the materials form a two-color stream at the junction, which travels down the leg of the “Y”. The emerging fluid droplets are all identical and are then “cured” to form solid microscopic particles.

By integrating any number of these microfluidic devices, Nisisako suggests it should be possible to scale-up the manufacture of Janus particles for commercial applications. He adds that their approach is not limited to polymer ingredients and making microparticles from other starting materials such as ceramics or metals should also be possible.

Advanced Materials

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