Making colourful glitter from...wood
Scientists are particularly interested in one ingredient of wood; cellulose. At the nanoscale, this material has incredible properties including bending light in a way that can give rise to amazing colours. The tiny pieces are called nanocrystals, and Iacopo Russo went to visit Silvia Vignolini, at Cambridge University’s Department of Chemistry, to see for myself how they are using nanocellulose to make glitter and pigments for cosmetics…
Sylvia - We are interested in cellulose because it's one of the two structural components together with lignin and while lignin is a polymer that has amorphous structure, the cellulose has a crystalline structure. And its crystallinity is what preserves as well, the original fibrillar type of architecture that you have in wood. This crystalline structure, it's important for the optical response because it imparts what is called the birefringence, but it's the way that interact with lights or birefringency means that the light can travel a different speed inside the structure itself and the traveling at a different speed is what is responsible of the structural colouration that we observe.
Iacopo - We're looking at a fume hood and there's some liquid inside, and a magnet is rotating around to mix it. What am I looking at?
Sylvia - You are looking at one of the processes that we have to do during the instruction of the cellulose nanocrystal. Because cellulose is something that you cannot synthesise, but you can only extract it from natural resources. We don't fully dissolve the structure of the cellulose, but you only remove the amorphous part until we get this nanocrystalline part.
Iacopo - So once we get that suspension, then what happens to it?
Sylvia - It can be used directly as an ink for printing into printers. We can use it to make larger laminate's of cellulose nanocrystal, we can use it to make other types of pigments. This is our starting material for a lot of our experiments.
Iacopo - Are there samples that I can have a look at?
Sylvia - Yeah. Let's go to the optics lab.
Iacopo - So we've just interrupted an experiment. Everything was dark in the room and I'm entering the optics lab. What happens to the nanocrystals once they deposit onto a substrate?
Sylvia - You have nanocrystal that are in suspension and depending on the concentration of the suspension, the interaction changes. If it's a really dilute suspension they don't interact. As soon as you increase the concentration, as soon as you force them to interact, reducing the volume of water, then you build up like a helical twist and having these different orientations is what creates the colour.
Iacopo - Okay. Let's have a look at some samples.
Sylvia - I'll want to show you a beautiful one even if the audience cannot see it, but you can see right. It's a really metallic colour and you think this cellulose is the same.
Iacopo - Depending on the angle at which we're looking at, I can see all colours of the rainbow.
Sylvia - This is essentially what is called iridescent. The colour changing function of the angle that you are observing. It's a characteristic of structural colour. Then we made it on this really large roll and deposited the large scale, and then it simply grinded.
Iacopo - It looks like glitter.
Sylvia - It looks like glitter. Depending on the size, the bigger they are makes them really glittery.
Iacopo - Okay. So that's the sparkly green and blue inside this liquid. But I can also see some bits of glass with a glittery substance on it. Some are red.
Sylvia - Essentially, it's the same of what you see here in the vial, but they spray deposited on a substrate.
Iacopo - It looks like a sky full of stars of different colours.
Sylvia - And then here, this one is multicoloured, but these other ones are also single colour. You see that they are all green.
Iacopo - And that's all to do with how the cellulose arranges in a different structure at the nanoscale, a very small scale.
Sylvia - That's true. The architecture is always the same. This helicoidal architecture combined with the birefringence of the cellulose is what interacts with the light and gets the colour. Then you can change the colour by changing some of the characteristics of the suspension. For example, adding salt, you can tune the colour, the other things you can tune the size of this particle when you grind it and then you have different types of effect, something that looks more homogeneous or something that looks more like glitter.
Iacopo - So what is the biggest advantage of using cellulose instead of currently used materials for pigments?
Sylvia - The main advantage is that you start with something that is the most abundant polymer that you have available on the planet. The fact that it's a biocompatible edible, and when you disperse it in the environment it has a really low toxicity or non-toxicity to different species. The convenience is just trying to have more sustainable components.