Morphing mirror could clear the skies for astronomers

2017-08-21 05:00:11

By Jeff Hecht (Image: ACS) Video: Ferrofluids – suspensions of magnetic particles in liquid – can be spectacularly manipulated using magentic fields, as in this artistic installation by Minkako Takeno You may appreciate the romantic twinkling of the stars, but the way Earth’s atmosphere distorts the light from celestial objects is a nightmare for astronomers, making them appear blurry. So a shape-shifting magnetic mirror made from antifreeze would be good news, making it easier to remove those twinkles and clear the view for ground-based telescopes. Mirrors able to change shape on command could also see uses elsewhere – such as in diagnosing eye disease. Ground-based telescopes currently use flexible mirrors with complex actuators underneath to correct for atmospheric distortion and keep a steady focus. They can even match the resolution of the Hubble Space Telescope. But the technology is expensive, and has a limited range of motion. Liquid mirrors are cheaper, and should be able to make larger corrections. The new morphing mirror developed at Laval University in Quebec is made from a magnetic liquid – a ferrofluid – that can be easily reshaped using magnetic fields. A video, top right, shows their amazing adaptability. Making mirrors from ferrofluids is not simple – they are not usually reflective and the normally oily liquid base can only with difficulty be coated with a reflective top layer of silver nanoparticles. Water-based ferrofluids hold the reflective layer more effectively, but water evaporates so quickly that the mirror could disappear within hours. Now Laval University chemist Anna Ritcey and colleagues have cracked those problems by suspending magnetic iron particles in ethylene glycol, the basis of automotive antifreeze. It can support a reflective film of silver nanoparticles and has negligible evaporation, creating an optical-quality surface that remains stable for more than 70 days, she says – in part thanks to a new coating added to stabilise the iron particles. Ferrofluid mirrors also have the advantage of not being limited to lying flat. The first telescope with a bowl of spinning mercury for a mirror will start working next year, but can only look straight up. However, with the right mix of magnetic fields, a ferrofluid mirror could even be held upside down if necessary. The extreme adaptability of magnetic mirrors could also be useful to ophthalmologists. A ferrofluid lens could be rapidly adjusted as necessary to compensate for the large distortions in diseased eyes during eye exams or treatment, Ritcey told New Scientist. But she says another major application could be in testing optics with aspheric surfaces, which are widely used in inexpensive cameras, and optical disk players because of their low cost and high performance. Such tests require comparison to a precisely matched reflective surface, reshaping a ferrofluid in an instant is better than having to build a whole new test surface. Journal reference: Chemical Materials (DOI: