Today's "Word of the Day" is ferrofluid. It's what chemists call a suspension of extremely small iron-based particles in some kind of liquid. The result is a room-temperature fluid with magnetic properties.
So why are you reading about this here and not in the Chemistry Junkies Forum? Because someday soon you might be ogling amazing images of deep space taken by a telescope with a ferrofluid mirror.
Astronomers have toyed with liquid-mirror telescopes for decades. In theory, a shallow vat of fluid, when spun slowly, assumes the paraboloidal shade needed for a reflecting telescope's primary mirror. This quirk of gravity is the key to the highly successful Steward Observatory Mirror Lab in Tucson, Arizona, where spinning ovens cast glass blanks for some of the world's largest telescopes.
The only suitably reflective liquid for spin-table mirrors has been mercury — not exactly the kind of material you want to spend any time with. Yet, despite mercury's toxicity, curious opticians continue to experiment with it. In fact, an international team is building a giant mercury-mirror telescope on a mountaintop in India, with the hope of getting "first light" sometime next year.
Yesterday I learned of a remarkable finding by three researchers at Université Laval in Québec, Canada. In their work with ferrofluids, Jean-Philippe Déry, Ermanno Borra, and Anna Ritcey have chanced upon a concoction consisting of ethylene glycol — the antifreeze in our cars — mixed with maghemite, an iron oxide. The maghemite particles are no more than 10 nanometers (100 angstroms) across, and they get coated with a type of acetic acid that prevents them from clumping together while in suspension.
The team's real breakthrough has been to add (by spraying) a small amount of similarly tiny silver particles. These then float atop the ferrodfluid to create a mirror-smooth surface that's more reflective than liquid mercury. Better still, the shape of the surface can be altered by placing electromagnets beneath the container and adjusting the voltage applied to them — no spinning is needed.
As the trio reports in the November 25th issue of Chemistry of Materials, a biweekly journal of the American Chemical Society, so far they've created a lab-bench liquid mirror 2.7 inches (7 cm) across.
What's most amazing is that its surface is accurate to 1/20 the wavelength of red light (624 nanometers) — easily accurate enough for telescopic optics. They haven't yet tried to deform the silvery surface into a paraboloid, but Anna Ritcey told me that she's confident that can be achieved with the right combination of electromagnets.
You won't likely see antifreeze-and-silver reflectors in backyard settings — among other things, they have to be pointed straight up. But it's certainly a technology that bears watching closely.