US researchers have used ferrofluids as liquid pistons that could be used to make adjustable liquid lenses with nearly perfect spherical interfaces for applications such as an optometrist's phoropter. A phoropter measures the way light is focused in the eye and is used to determine prescriptions for glasses and contact lenses. Ferrofluids are colloidal solutions of ferromagnetic nanoparticles suspended in a dispersing liquid. Ferrofluid droplets can be manipulated by a magnetic field, so they could be used in systems that need precise control, such as optics, drug delivery, and electronic devices. Amir Hirsa and colleagues from Rensselaer Polytechnic Institute in Troy, New...