Metamaterials with effective negative indices of refraction offer the possibility of making invisibility cloaks by routing light around a concealed object. A new approach on this theme is based on a novel form of ferrofluid. The idea by Y Gao of Fudai University in Shanghai and the Chinese University of Hong Kong and colleagues is to use silver-coated iron-oxide nanoparticles suspended in water.
An external magnetic field would cause the particles to line up into chains so that the desired metamaterial would form by self-organization. Experimental demonstration of the technique still remains to be done, but the idea is quite new in the field, which has so far concentrated on solid devices. Maybe this is what the author J K Rowling had in mind in the Harry Potter novels, where she describes invisibility cloaks as “fluid and silvery".