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Tag: plasmonics

Like disappearing ink, but cooler: Laser-powered invisible images demonstrated

Enlarge / In the UK, Jean Michel Jarre performs on stage at NIA Arena on May 24, 2009. (credit: Steve Thorne/Redferns/Getty Images)

I think everyone is aware of the trick with invisible ink. Write your message in lemon juice on paper, and when the juice dries it cannot be seen. But if you heat the paper, the lemon juice reacts with it and turns brown, bringing forth your shining prose for all to read.

That's so old school—I want laser powered invisible writing (and, no, I am not paid to make sense. Why do you ask?). Since lasers are what make life worthwhile, others evidently felt the same. Lo and behold, there has now been laser-powered invisible writing.

Watching glass glow

To create laser powered invisible writing, we need to delve into how light interacts with matter. Imagine a glass plate. If I shine a laser through the glass plate, pretty much nothing seems to happen. But internally, there is a whole lot going on. The electric field from the laser beam grabs hold of the electrons surrounding the atoms in the glass and gives them a good shaking. As the electrons shake up and down, they absorb and re-emit light from the laser. The color doesn't change, but the light slows down a little.

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Measuring viscosity with tiny golden antennas

(credit: Universal Images Group / Getty Images)

I was kind of shocked and amazed by a recent publication in NanoLetters. It seems that viscosity measurements are still difficult. In my ignorance, I had assumed that this was a solved problem. And, just to show the depths of my ignorance, it turns out that you can learn something about a person's health by measuring the viscosity of their blood. This process is time consuming, as I'll explain in a moment. Now, thanks to the power of our ability to build little gold-iron alloy helices, these measurements just got a whole lot easier.

Stick around and I'll tell you about viscosity

Viscosity is a measure of how well things flow. So, for instance, water flows quite easily and rapidly, while some oils flow more slowly—we say that oil is more viscous than water. The study of fluid flow is a very complicated business so, to simplify the problem, you have to ask yourself what is important. For instance, water flowing down a river is probably dominated by the sheer mass of moving fluid. That means you can probably ignore any influence of viscosity and just worry about mass.

On the other hand, when blood reaches the extremities of the body, it is flowing in very fine channels. There is not a lot of mass to the fluid, but the viscous forces between the channel wall and the fluid are enormous. Here, it might be appropriate to ignore anything to do with mass and focus on viscosity. By examining relationships like this, you can build reasonably accurate models of fluid flows without enduring the pain of the full complexity of fluid dynamics equations. But you can only choose what to include in your models if you know the viscosity reasonably accurately.

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