Stuff that gets thicker with light

Like you, I enjoy learning about entirely new classes of materials.

Wait--you don't enjoy that? Dude. Seriously.

This week the Journal of the American Chemical Society has an article called "A Simple Class of Photorheological Fluids: Surfactant Solutions with Viscosity Tunable by Light." Here's what that means: The authors, researchers at the University of Maryland, have figured out how to make relatively cheap, thick gunk that gets thinner when exposed to ultraviolet light.

Already cool are electrorheological and magnetorheological fluids. These change viscosity—they get thicker or thinner—in electrical or magnetic fields. And they're commercialized. Here's a pretty cool article from 2002 about using an MR fluid as a vibration damper in washing machines, to keep them from "walking" during an imbalanced spin cycle. And here's Wikipedia on ER, explaining that the stuff's been proposed for use in automotive shock absorbers. As I understand it, both MR and ER fluids tend to be colloidal suspensions, a solid mixed into a liquid. Hit it with a field and the solid changes conformation, thickening the mix.

So here's what the paper's authors, led by Srinivasa Raghavan, say about their new formula for photorheological fluid:

ER and MR fluids have been around for more than 50 years, and their ability to switch rheological states has formed the basis for technologies such as dampers, clutches, and valves. However, these fluids are typically two-phase systems that tend to aggregate or settle with time. PR fluids, on the other hand, promise to be true, single-phase solutions, structured at the nanoscale. Also, an advantage of light over electric or magnetic fields is that light can be directed at a precise spatial location; this becomes especially valuable in microscale or nanoscale applications. Accordingly, PR fluids could be an enabling technology for microfluidic or MEMS devices and sensors.

So...that's neat. Shine UV on this gunk—which the team says any reasonably well-equipped lab can make for pretty cheap—and it viscosity drops by four orders of magnitude. It goes from a gel to water. Caveat: The particular formula in question here (cetyl trimethylammonium bromide and *trans-ortho-*methoxycinnamic acid, just as you probably suspected) isn't reversibly rheological. Like, you can just hit it the thin version with infrared light and have it get thick again.

But if you could? Ooh, how about loading it up with some pigment for a car that's cherry-red during the day and then matte-black at night? Or...suntan lotion? Put on an imperceptibly thin coat that thickens when you hit the sun? Actually, that could be kind of gross. Or...really interestingly gooey blacklight posters? Honestly I don't get what the use would be in microfluidics or microelectromechanical systems. But I'm open to suggestions. After all, new classes of materials don't always come with instructions.

UPDATE 2/7/07 10:40 AM Commenter "James" writes:

MEMs applications--think valves. Shine light where/when you want no flow (or slow flow) and take it away (or shine new light) when you want it to go again.

Cool idea, though it seems to me you'd end up with a packaging problem: really big light source or light pipe + really teeny tiny MEMS chip.

And commenter "Becky Oskin" points me toward a North Carolina company called Liquidia, which (according to this story in the Raleigh-Durham News-Observer) is trying to commercialize a material called Fluorocur. It's liquid at room temperature and turns solid when exposed to ultraviolet. (Perhaps that Becky Oskin is the Becky Oskin who works in the press office at Duke University; Fluorocur appears to be a discovery of researchers at the University of North Carolina at Chapel Hill.)