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A colloidal crystal is a regular arrangement of tiny particles suspended in a liquid. University of Pennsylvania researchers created 2D "crystallites" of colloidal particles, and in the process the structure seemingly defied Coulomb's law, which says that opposite charges attract each other.
The top image is a graphic of latex spheres which form a 2D crystallite on a "bilayer vesicle," an organic structure similar to the membranes which surround living cells. A few negatively charged beads attach to the positively charged vesicle. But then, the vesicle repels additional beads.
The lower image shows the self-assembly of 1-micron latex spheres onto the positively-charged vesicle. The repulsion of the additional beads results from the initial beads bringing about the migration of electrical charges in the vicinity of the bilayer vesicle. This charge migration makes the vesicle effectively negatively charged in some places away from the initially attached beads. The 2D colloids offer intriguing possibilities as templates for artificial biomaterials and industrial catalysts ( Images courtesy Dave Weitz, Laurence Ramos, and colleagues at the University of Pennsylvania).
reported by: Ramos et al. 17 December 1999 and Aranda-Espinoza et al., 16 July, 1999, Science