An unexpected phenomenon observed by physicists at Southern Illinois University may have implications for superconducting thin films and unusual forms of wetting.
Rongjia Tao and his colleagues began by wanting to observe the motion of micron-sized copper oxide (e.g., Br-Sr-Ca-Cu-O) superconducting particles (suspended in liquid nitrogen) in an electric field running between two electrodes. As an electric field helps to define a preferred direction in space, metal particles in this situation would be expected to bounce between the two electrodes or line up. The superconducting particles ignored this hint and, to the researchers’ great surprise, formed themselves into a ball. The ball, about .25 mm across and containing over a million particles, formed quickly and was quite sturdy, surviving constant collisions with the electrodes.
Tao and his collaborator, Princeton theorist Philip Anderson, have concluded that the effect is an artifact perhaps something to do with the way in which the surface energy of the particle ensemble is reduced by self-assembly into a ball. This unprecedented new surface energy is related to the acquired surface charges on the particles and the reactions among the layers of the balls.
The process seems to be a product of superconductivity (the same particles, above their superconducting transition temperature, do not ball up but instead queue into lines). Granular properties of the particles might also play a role in the process and in the ball’s internal structure. Further experiments are planned in the microgravity of space, as inter-particle interactions are mitigated at present by the liquid nitrogen needed in the experiment to neutralize gravity.