Brewster's angle is the angle at which light of a certain polarisation won't reflect off a surface. The resulting reflection will then be made up of only light from a single polarisation (p-polarised). This little optical quirk is how polarised lenses remove lots of scattered reflections in photographs and why polarised sunglasses are so much better when lounging by the seaside. However, rather than just being something that makes sunny days even better, it also provides a neat trick for visualising things that would otherwise be invisible to other analytical methods.
Brewster's angle for water is approximately 53.1°, which is calculated from knowing the refractive index of the water and the surrounding air. If either one of these values was to change, then the angle would also change. For example, if your measured the Brewster's angle of a film of olive oil (refractive index of ~1.46) you'd get around 55.7° which is quite a big shift (relative to most things in optics).
Now, if you set up a light source and camera at exactly 53.1° over a tank of water and block all the s-polarised light bouncing off the surface into the camera then all you will see is black. If you introduce to the water surface a contaminate, such as oil, this will change the Brewster's angle and so the water will begin to reflect p-polarised light, which will pass straight through the s-polarised filter and into the camera. So, from the camera's point of view, the water will appear black and the oil will appear bright. This will work no matter how thinly the material is spread, so if you have just a single molecule thick layer (~2 nm which is around the same thickness as a single strand of DNA) this will still show up nice and bright on the camera. (this is much clearer in the video but we'll get to that in a moment).
Looking at materials like this provides a wealth of information to scientists on the way materials interact with each other or with the water beneath them. In the main this is used as a tool for examining materials for further use in applications such as benzene sensing or medical device coatings or examining a range of novel exotic materials. However it is not a commonly used technique, one reason for which is cost of a Brewster Angle Microscope (BAM). Recent developments in Micro-BAMs have brought the cost down but a BAM currently costs anything from £20,000 - £75,000, which is serious money. When I did this work I was a lowly PhD student with purchasing authority for the grand sum of £0, which is some way off £20,000, so I realised that if I wanted some nice pictures of my materials then I would need to get creative.
The photograph above shows the second generation design of my home made BAM. The first design was made using laboratory retort stands which just about worked but the lack of gearing, motors or align-able parts meant it took a hour just to get a poor quality fuzzy image. The second generation has the major advantage of being made with Lego. A vital part of the BAM is getting the laser (1 mW green laser pen from SP3Plus Ltd) and the camera ( VMS-001 Veho, bought second hand) to align correctly. This was big problem in the hand-adjusted version because, not only were we trying to get the angle right, we were also trying to line up the the laser and the microscope. Lego neatly solved this problem by having such insanely high tolerances (~10 µm) on it's parts, so I knew that if I build two identical frames and mounted the laser in the centre of one and the camera in the centre of the other they would aligned well enough to get a 0.5 mm beam into a 2.5 mm camera aperture. The frames shown also include motors and gearing to allow for simple changing of the angle of the camera or laser. These motors were re-calibrated at the start of every experiment so that specific changes could be made for an accurate measurement of the Brewster's angle for the material being used. Attaching the laser and the camera to the frames was a little trickier as unfortunately neither Veoh or Sp3 make their respective kit with lego mounting or dimensions in mind. Conveniently, the Sp3 laser did come with a mounting rig which had mounting pins on the bottom plate, by chance these pins were within 0.5 mm of the required positions to simply drop them in to lego frame, this was quickly rectified with some sand paper. The camera however didn't come with anything and was mounted using a V-shpaed holder this ensured that the centre of the camera lined up with the laser and it also allowed for an additional mount for a polarising lens to cover the camera aperture. More photographs of the finished system can be found here and here.
Finally once it was all set up the system was tested with some stearic acid (a common fatty acid) and it produced the video shown below.
As I mentioned before, the white shapes in this video are the stearic acid material floating on the black background. The shapes shown will naturally move around in the air currents in the lab and any residual currents in the water the material is spread on. The material spread in this video has been prepared in such a way that the film shown is only 1 molecule thick. In some regions it may be more than this and these show up brighter than the majority of the film.
The microscope is currently undergoing a re-design and I hope that the 3rd generation will produce even clear images (auto focusing) over a wider area. I am also quietly optimistic that I can also motorise the polarising lenses, which would allow me to collect much more data on each material. Once improved I'll publish a follow up set of results here. In the mean time you can always follow me on twitter (@MCeeP) to get some more detail on this and other work we do.