Can conductive polymers generate eddy currents

Mini-scale donuts: colloid crystals as a mold for nanorings

Technicians and scientists are fascinated by the almost magical properties of ring-shaped objects. For example, magnetic fields in conductive rings can create a permanent eddy current, and cleverly mixed rods and rings act as coils and antennas, resulting in materials with amazing optical properties, such as a negative index of refraction. However, these phenomena are size dependent. In particular, if the rings are to interact with visible light, their size must be adjustable down to the sub-micrometer range. But how do you make such tiny rings? Werner Goedel and Feng Yan from the TU Chemnitz have developed a new "three-dimensional" strategy in which so-called colloid crystals are used as casting molds.

First, just micrometer-sized spheres are slurried in a solvent and pressed together in a centrifuge to form a regular three-dimensional package, the colloid crystal, and then dried to form a "mold". In the next step, the scientists infiltrate the mold with a polymer solution, for example polystyrene in chloroform. The special trick: the form must not be completely filled, only partially. The liquid does not sag down, but collects in the tiny gaps around the contact points between the spheres and forms small rings there. This phenomenon is caused by capillary forces. If the solvent evaporates, the polystyrene remains in the crevices as a solid. In the last step, the researchers dissolve the beads. What remains are tiny, donut-like polystyrene rings of a uniform shape and size. The rings can be made from a number of different materials and their size can be easily adjusted using the diameter of the beads.

The researchers succeeded in producing rings from different polymers and ceramics and varying their outer diameter between about 400 nm and 150 nm, the inner diameter being in the range from 150 nm to 50 nm.