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Asked by anon-255889 to Viktor, Jose Angel, Adam on 28 May 2020.
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Viktor Doychinov answered on 28 May 2020:
I find that difficult to answer to be honest. One of the good things about working as a researcher is that you get to do a lot of various things. I have enjoyed different things in the different projects I have worked on.
But, if I have to pick a project, I will pick one that some colleagues of mine and I are trying to start now. It is about trying to detect and count how many worms there are in a particular block of soil. We want to do this because worms can tell us how healthy the soil is, and this is important for growing crops for food. We also want to do it in a way that does not disturb the soil – it would be quite easy to just dig it out and count the worms by hand, but then you are left with a hole in the ground.
It is quite interesting, because it is a mix of disciplines – you have physics, engineering, biology, chemistry, data analysis, even radio astronomy! I find it is a lot more fun to work with people from other backgrounds.
I also quite enjoyed working on a personal project for my pets. I wanted to make sure they are neither too cold nor too warm, so I put together some electronics to measure the temperature in their vivariums and to send me a text if it needs adjusting.
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Adam Washington answered on 8 Jun 2020:
My favourite project is still my research on bird feathers, which might seem out of place for a physicist. As a bit of background, most colors that we see in nature come from pigments in a material. These chemicals absorb certain colors of light, but the rest of the other colours are bounced back, creating the result that we see with our eyes. There is a second way of making colours, though, called structural colour. Here, a transparent material has pits and gaps in it that are around the same size as light itself. Every colour in the rainbow has a certain size associated with it, called the wavelength. If the wavelength of the light is the same size as the gaps, the light bounces off the gaps instead of passing through.
This structural colour has a huge advantage that the colour is created by the geometry and not the chemistry. As light continually bounces off of a pigment, the chemical can become unstable and break apart, like a surfer’s hair being bleached by the sun. The geometry can’t fail like this and materials based on structural colour can maintain their colour far, far longer than ones based on pigments and dyes.
The bird feathers came into the picture because many species of birds have a structural colour in their feather to produce blue and ultra-violet colours. However, we had a few open ended questions from this. First, how are they making these sorts of structures in their wings? Can we make a similar sort of material? And why do they only make blue and ultra-violet? Why do all of the red and green birds use pigments instead of structures?
Our experiments pointed us toward a process of “spinodal decomposition”. Think of a bottle with oil and water in it. Normally, you just have the oil sitting atop the water, but we will not violently shake our imaginary bottle. We now have many tiny droplets of oil and water mixed together. If we were really bored, we could sit and watch those droplets in the bottle. The smaller oil droplets would come together, forming a single larger droplet, and the same would happen to the water. If we wait long enough, we would eventually have the oil floating on top of the water again. This is the decomposition.
Now, imagine that we could freeze the process somewhere along the way. We know that we started with a lot of small droplets, but that we’ll also start seeing larger and larger droplets the longer that we wait until the freeze the process. The existence of the larger droplets doesn’t mean that all of the small droplets have been combined yet, but just that the larger droplets are there.
This is similar to the process we believe occurs within the bird feathers. The keratin (basically hair or fingernail material) that makes up the feather barb starts a decomposition within water, then the reaction it frozen and the water is removed, leaving keratin and air. There will always be the smallest air bubbles, so the ultra-violet and blue colours are always present. If the feather barb waited a while before the reaction was stopped, then there are some large pits that reflect green light. However, the smaller pits are still there, so both blue and green are reflected, giving something of a cyan colour. If we waited an even longer time, then there are pits large enough to reflect red light as well. To the human eye, if something reflects red, green, and blue, then it appears white.
In nature, we can find examples of structural feathers that are blue, cyan, and white, but no other colours. Similarly, we performed x-ray measurements on feather barbs from multiple different bird species known to exhibit structural colours and the internal structure was always consistent with what we would expect from spinodal decomposition. One of my colleagues on the project even developed a process of creating structural material from transparent cellulose.
Before I finish, I should say that there’s a huge caveat in all of this. If you look at the neck of a pigeon, you’ll see flashes of green and red which are absolutely structural. That’s a process of iridescence, where the colour will change depend on the angle of the light. Our interest was only in “diffuse” structural colour, where the colour is the same regardless of the angle between your eye and the light. It is diffuse structural colour that cannot make green or red feathers. The iridescence will make every colour, but only for particular directions.
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