Tiny marbles, black on one side and coloured on the other, can be made by 'curing' suspensions of silica particles with an ultraviolet lamp, according to Seung-Man Yang and his colleagues at the Korea Advanced Institute of Science and Technology in Daejeon. When an electric field is applied, the marbles line up so that the black sides all face upwards, which suggests they may prove useful pigments for flexible electronic displays.
The researchers suspended a flow of carbon-black particles mixed with silica and a transparent or coloured silica flow in a resin that polymerizes under ultraviolet light. They then passed the mixture through a tiny see-through tube. The light solidified the silica and resin as balls with differently coloured regions (pictured), each about 200 micrometres in diameter.