Japanese scientists have developed a rotating motor that sits on a droplet and spins when an electric field is applied - a feature that could be used in optical devices. 

Transparent fluids with high refractive indexes are used in lab-on-a chip optical systems to control the angle or movement of micro-plates. Horizontal and vertical movements have been achieved but so far rotational movement using fluids has not been possible. Since diffractive gratings or polarisers often activate or switch their functions in rotational positions, a method to rotate microstructures is required.

Now, Atsushi Takei, Kiyoshi Matsumoto and Isao Shimoyama at The University of Tokyo, have used electrowetting to cause a floating plate to turn. A droplet is placed on a base plate and another non-circular plate floats on top. Electrodes patterned with an annular shape on the base plate surround the droplet. By changing the voltage applied to the electrodes, the bottom edge of the droplet is deformed, creating torque between the droplet and the plate, causing the plate to spin.
 

The turning motion is created by applying an electric field to the droplet

'This is a unique mechanism that Takei and colleagues have developed,' comments Chang-Jin Kim, an expert in microengineering at the University of California at Los Angeles, US.

Takei emphasises that an advantage of this arrangement is that the various parts can be made of a transparent material, so it can be used in optical systems. Takei and his group intend to try and make smaller motors, the smaller the scale the more difficult it is to control the forces on the droplet so this is a challenge for the future, he adds.

Holly Sheahan 

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