Each fluid reveals a different letter.
What’s the News: Scientists have developed a chip that can instantaneously identify fluids applied to it, just from their unique surface tension. In a handheld device, it could help toxic site remediators figure out what that ominous clear liquid is. And there’s a bonus for the kids-in-the-treehouse user demographic: different secret messages can appear on the chip depending on what fluid is applied.
What’s the Context:
- Materials scientists—who, put simply, study stuff—have long been interested in how fluid moves through the tiny myriad holes of substances like coral, or bone.
- By building coral-like structures out of glass in the lab and then treating them with various chemicals, they can make it impossible for some fluids to seep into the maze of holes, while others glide right in. A substance’s ability to absorb fluids is called “wettability,” and it’s the subject of much research, as you can imagine—it’s the reason your raincoat stays dry and your sponge sops up spilled juice.
- A fluid’s ability to seep through holes is dependent on its own physics, as well as the wettability of the substance. Molasses, for instance, is too thick to creep into holes smaller than a certain size, whereas ethanol can slip through most anything. Fluids’ surface tension—the characteristic that lets them form droplets—is particularly important to their ability to infiltrate a substance, and that’s the quality these researchers used to distinguish fluids on their chip.
Decreasing the concentration of ethanol causes new letters to appear.
How the Heck:
- In this instance, researchers made a chip out of a coral-like glass, then began applying a series of stencils to it.
- By putting a stencil in the shape of a letter on the chip’s surface and exposing the material around it to a certain chemical, they created a difference in the wettability of the letter versus the rest of the chip. They did this several times with different shapes and different chemicals, so each letter or symbol had its own specific preferences in terms of what fluid—with what amount of surface tension—it would absorb.
- When the chip was dry, the surface was shiny and unmarked. But when they wiped ethanol over it, the letter I shone out. The surface around it had absorbed the mixture, turning dark, while the letter stayed dry. Just slightly decreasing the concentration of ethanol made another letter appear, and then another and another, spelling out “W-INK,” the team’s name for their invention, Watermark Ink.
- In an industrial context, the stencils could be tiny, so a chip that had been treated with a variety of chemicals could distinguish between a large number of fluids.
The Future Holds: The system is remarkably sensitive, as demonstrated by its ability to tell apart different concentrations of ethanol. The team thinks it could be used by officials checking different varieties of gasoline for purity and by environmental cleanup workers in the field—and the possibilities for chemical encryption are certainly nothing to sneeze at. Perhaps in the future, spies will be advised to keep a gallon of ethanol handy to decrypt their instructions.
Reference: Encoding complex wettability patterns in chemically functionalized 3D photonic crystals. I.B. Burgess, L. Mishchenko, B.D. Hatton, M. Kolle, M. Loncar, J. Aizenberg. J. Am. Chem. Soc. (2011)