Shaking electronic structures

The progression toward smaller and smaller electrical and mechanical components presents challenges in creating devices on scales measured in microns and nanometers. One solution may be to develop materials that automatically arrange themselves in useful patterns.

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A collaboration of researchers at Argonne National Laboratory and Institute of Physics for Microstrctures of the Russian Academy of Sciences has developed a new method for encouraging microscopic particles to self assemble into desired complex patterns. The technique is inspired by the patterns formed in shaken mixtures of much larger granular materials.

Rather than mechanically agitating tiny grains to create self assembled patterns, the method relies on electrostatic fields to drive metallic microparticles immersed in liquids. The researchers placed 120-micron bronze spheres in a mixture of toluene and ethanol trapped between glass plates. The plates were coated with thin layers of transparent conducting material, and an electric field of up to 3 kilovolts per millimeter was applied between them.

Particles that contacted the lower plate acquired a charge and were repelled toward the upper plate. If the upward electrostatic force is sufficient to overcome gravity, the particles fly upward, contact the upper plate where their charge is reversed, and then are forced back down again. In effect, the alternating charge on the particles is analogous to shaking a container of macroscopic grains.

As in the classic granular material experiments, varying the conditions causes the particles to form vortices, pulsating rings, honeycomb patterns or other structures.

Ultimately, say the researchers, studies such as this may allow them design systems that spontaneously self assemble into useful structures on increasingly tiny scales.