Magnetics drives particle patterns

Researchers from Argonne National Laboratory have devised a way to use electric and magnetic fields to assemble magnetic microparticles into a wide variety of patterns, including clusters, rings, chains and networks.

Particular shapes depend on the amplitude, or strength, and frequency, or vibration rate, of the magnetic field.

In general, low-frequency magnetic fields make the microparticles form compact clusters, while high frequencies drive the microparticles into chains and net-like patterns.

The researchers' work is one of several efforts aimed at finding ways to cause particles to self-assemble in order to make microscopic machines, smaller electronics and materials with precisely tuned properties, and to do so relatively cheaply.

The researchers' method could also be used to organize biological particles like viruses and bacteria that are tagged with magnetic nanoparticles. This is one step toward biomedical tools that involve precisely controlling the propagation of bioparticles through capillaries, according to the researchers.

The researchers' prototype consists of a pair of magnetic coils and a pair of horizontal plates sandwiching 90-micron-diameter nickel particles. A micron is one thousandth of a millimeter.

The researchers caused the particles to move by generating electric and magnetic fields around the particles. They generated the electric field by applying a voltage to the plates, and the magnetic fields by applying a voltage to the coils. One coil produces a constant magnetic field while the other produces an alternating field.

The nickel particles formed patterns depending on the strength and frequency of the alternating magnetic field. At a frequency of 20 hertz, or cycles per second, the particles formed clusters, at 50 hertz they formed net-like patterns, and at 100 hertz they formed chains.

The work appeared in the March 18, 2005 issue of Physical Review Letters (Structure Formation in Electromagnetically Driven Granular Media).