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Arthur T Knackerbracket has found the following story:

In radio terms, the use of an helical antenna is well understood and it works well. A completely different - and apparently unrelated - problem is how to measure the viscosity of blood which is complicated by the need to make the measurement in the main arteries and veins, and also in the much smaller capillaries. In these 2 cases the science behind the viscosity is completely different.

This story is about someone who had a bright idea about how to solve this problem:

The basic idea behind the new work is to measure the viscosity of the fluid between the blood cells, which requires a probe that is much smaller than the cells themselves. Enter plasmonic antennas. Plasmonic antennas are little dollops of metal, somewhere between 20 and 200nm (10^-19m) in size. These metallic blobs respond fiercely to light.

This response depends on them being metal. The reason a metal is reflective is that its electrons are free to move around in response to light. When a light field hits the metal, the electrons re-radiate its electric field by jiggling up and down in response. Normally, this behavior is averaged out over any substantial hunk of metal.

If the metal is just a tiny blob, then the electrons are restricted in their motion: they pile up at one end of the blob and slosh back to the other. In doing so, they create an enormous electric field, so they radiate lots of light in every direction. As a result, little gold particles scatter huge amounts of light.

[Continues...]

Things are never quite that simple, but by a clever bit of lateral thinking the problem was solved:

And this is where the researchers got clever. Instead of making gold spheres or bars, the researchers made tiny spirals that were just 170nm long and 50nm wide. These antennas will scatter light, too. But their response is strongly dependent on the polarization of the light.

[...] These are linear polarizations. However, the electric field might not do any of that. Instead, the size of the electric field might remain the same, but the direction it is oriented rotates either clockwise or counterclockwise. In this form, called circularly polarized light, it looks like a corkscrew coming to extract you from your front row seat.

When the wavelength of light for this circular polarization matches the pitch of the helix, the helix will scatter very strongly. But it will only respond if two conditions are met. The direction of the helix and the direction of rotation of the circularly polarized light have to match. And, the helix needs to meet the light head on. If you send in light that is a mixture of the two circular polarizations, one polarization scatters back very strongly and the other does not. This can be measured and used to determine the orientation of the antenna relative to the direction the light was traveling. And, the difference in scattering for the different polarization is not very strongly influenced by the cells floating around in the blood.

The helical shape gives the researchers a signal that is unique to their antenna, and it depends on the alignment of the particle relative to the incoming light. So it necessarily tells you something about the antenna's location.

A viscosity measurement is made by rotating the magnetic field. The antennas will respond, continuously lining up to the magnetic field. But, thanks to the drag due to the fluid's viscosity, they will always lag slightly. Measuring the lag allows the viscosity to be calculated.

This summary cannot do justice to the full article and I encourage you to read it in full.

Original Submission