Spherical agglomeration of superconducting and normal microparticles with and without applied electric field

It was reported by Tao et al. [Phys. Rev. Lett. 83, 5575 (1999); Physica C 377, 357 (2002); Phys. Rev. B 68, 144505 (2003); Physica C 398, 78 (2003)] that in the presence of a strong electric field superconducting microparticles assemble into balls of macroscopic dimensions. Such a finding has potentially important implications for the understanding of the fundamental physics of superconductors. However, we report here the results of experimental studies showing that (i) ball formation also occurs in the absence of an applied electric field, (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature, and (iii) it can also occur for nonsuperconducting materials. Possible origins of the phenomenon are discussed.