Applied Physics

Dropping the Voltage

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Science  09 May 2003:
Vol. 300, Issue 5621, pp. 867
DOI: 10.1126/science.300.5621.867b

“Smart windows” that control the amount of light coming into a room are based on nematic liquid crystals dispersed in polymer matrices. With no applied electric field, the random orientation of the molecules scatters light and the display is dark. Turning on the field aligns the molecules, and in this state they match the refractive index of the matrix and transmit light. This same idea can be used to control the direction of the transmitted light in device applications, but even higher switching voltages are required. Rudhardt et al. found a way to disperse nematic liquid crystals that can lower the necessary voltage. They created droplets of a nematic by extruding the material through a capillary into a flowing stream of water that contained a small amount of poly(vinyl alcohol) (PVA). Because the droplets formed when the viscous drag exceeded the surface tension, they tended to be very uniform in size. The droplets were placed on a conducting glass slide (one coated with indium tin oxide) and allowed to dry. The droplets spontaneously ordered into a hexagonally close-packed monolayer, but the thin PVA layer prevented coalescence. The PVA also controlled the orientation of the nematic, which at rest is governed by two disclinations at the edges. The devices can be switched between an off state to an ordered one at 0.05 V per micrometer, but a transition between two ordered states which changes the diffracted intensity by 80% can be achieved at voltages as low as 0.03 V per micrometer.—MSL

Appl. Phys. Lett. 82, 2611 (2003).

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