Ferrofluid research, close-up. A ferrofluid is a liquid mixed with small metallic particles that can become magnetized in the presence of a magnetic field. Ferrofluids are used in electronics, spacecraft, and medicine, but are also a fascinating way to visualize a magnetic field in three dimensions. A ferrofluid is known for having properties of two different states matter: liquid and solid. Whether it's a liquid or solid depends upon whether a magnetic fields is present. This ferrofluid is being studied at Princeton Plasma Physics Laboratory (PPPL), New Jersey, USA. Photographed on 10 October 2011.