Shaken Not Stirred

The progression toward smaller and smaller electrical and mechanical components presents tremendous challenges to engineers and scientists as they strive to create devices on scales measured in microns and nanometers. One solution may be to develop materials that automatically arrange themselves in useful patterns. Now a collaboration of researchers (Igor Aronson, 630-252-9725) at Argonne National Laboratory and Institute of Physics for Microstructures 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.

Numerous, beautiful experiments involving agitated containers of sand, ball bearings, or other granular materials have shown that the combination of gravity and inter-particle forces from collisions can lead to a rich variety of patterns, ranging from particle-like localized excitations known as oscillons to honeycomb shapes to chaotic swirls (Update 264). Other studies have helped to explain why large and heavy brazil nuts sometimes rise to the top in shaken containers of mixed nuts (Update 132). The new research extends such experiments into microscopic regimes.

Rather than mechanically agitating tiny grains to create self assembled patterns, however, 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 (see figure). Ultimately, say the researchers, studies such as this may allow us to design systems that spontaneously self assemble into useful structures on increasingly tiny scales. (M. V. Sapozhnikov et al., Physical Review Letters, 21 March 2003)