Veritalk is a podcast produced at Harvard’s Graduate School of Arts and Sciences. In each three-episode miniseries of Veritalk, you’ll hear how PhD students from different fields are trying to answer really big questions about the world.
Victoria Hwang and Annie Stephenson, PhD students in applied physics, use bird plumage as the inspiration for their work on structural color. It turns out that replicating some of nature's most impressive colors is actually pretty tricky.
I mean, why?
ANNA FISHER-PINKERT: From the Harvard Graduate School of Arts and Sciences, you’re listening to Veritalk. Your window into the minds of PhDs at Harvard University.
I was curious. . . I’ve always wondered. . . why?
This is our second episode about Plumage: From birds to bling – the very deep reasons behind the most superficial things on earth.
Episode 2: THE WILD BLUE YONDER
I really encourage you to go back and listen to the first episode about plumage, because I had a great talk with Dakota McCoy, a PhD student studying the blacker-than-black feathers that help male birds of paradise stand out to potential mates.
DAKOTA MCCOY: We started trying to understand first, how is it making such an incredibly dark black such an absorbent surface? And then also why — you know usually in this kind of evolution you see beautiful colors you don't really see black, where there is no color at all.
AFP: Last week, we talked about why a bird might want specially colored feathers. But this week, we’re going to talk a little more about how a bird might make super black feathers. Or blue feathers. Or iridescent feathers. And that means we need to talk about the physics of color. So I asked two PhD students in applied physics to meet me in the studio.
ANNIE STEPHENSON: Hi, my name is Annie Stephenson, and I am third year PhD student in the applied physics program at Harvard.
VICTORIA HWANG: Hi, I'm Victoria Hwang. I'm a fourth year PhD student in applied physics at Harvard.
AFP: Victoria and Annie study structural color. And they actually make color in the lab. They brought in vials of blue, green, and purple pebbles or liquids. These are samples that they make in their lab – and they’re incredibly beautiful.
AS: It's nice to be surrounded by things that are kind of artistic in your daily life as a scientist.
AFP: Everything in the structural color lab is inspired by nature. If you visit the lab, you’ll know which door is theirs because someone has taped up a picture of a brilliant turquoise bird with bright magenta feathers on its throat.
VH: The mascot in our lab is the blue cotinga bird. It's a very pretty bird, it's blue, and, as contrary to other species of birds that you see, it's a very homogeneous matte-looking blue that is very rare in nature. That color comes exclusively from structural color.
AFP: Most of the colorful things you find in nature – red cardinal feathers, orange tiger stripes, yellow sunflowers – appear colorful to us because they contain pigments that absorb some wavelengths of light and reflect others. But structural color, as we learned last week, is a little different.
VH: Structural color is different from other colors that we see in nature in the sense that in nature we see a lot of examples of absorbing pigments or dyes. So a lot of the birds and species that you see that are red or that are yellow, for instance, have this property that they absorb light — and all the light that does not get absorbed — that's the light that you see. So red birds absorb blue colors and green colors, and so you only see the red that is remaining. Structural color, on the other hand, it reflects preferentially some colors, and it transmits the rest. So if you see a blue structural color, it means that all the red colors and the green colors are just passing through the material and you only end up with the blue that is reflected back to your eye.
AFP: Annie showed me a peacock feather to illustrate Victoria’s point.
AS: So if you take a look at a peacock feather, you can see that it has this very brilliant bright green color and some blues and oranges. And when you change the angle of the feather while you're looking at it, the colors change slightly. They change the hue of the color. And this is one of the main reasons that we know that this is structural color.
AFP: If you hold a peacock feather up to the light and turn it around, the colors really do change from dull yellow-green to a bright teal-green.
AS: So, whenever we have this kind of iridescence, or angle-dependence of color, it's due to the fact that when you have a nano-scale structure, and you change the angle that you look at it, you're changing how the light travels through the material, and therefore changing the color that you see.
AFP: The color-changing property of the peacock feather is one clue that we’re looking at structural color, rather than color made through pigments. Structural color shows up in a lot of different places in nature.
AS: We also see structural color and beetle shells, and butterfly wings, and even certain plants. But in particular, we are the most interested in a lot of the structural color that we see in bird feathers because of this wide variety of structures that we see and how it relates to the structures we make in our lab.
AFP: Birds make structural color in their feathers using keratin, but Annie and Victoria can make structural color in the lab using plastics.
AS: In our lab, we start with a suspension of these nanoparticles, and it's called a colloid. And it basically looks exactly like milk. And that's because milk is actually an example of a colloid that we see in nature.
AFP: The colloid is white, like milk, because it scatters light at all wavelengths.
AS: So, we have our colloidal suspensions of these nanoparticles. And we start by taking that suspension and putting it in a centrifuge for a long time. And in a centrifuge you just spin the particles around at a very high speed, and it causes the particles to pack densely at the bottom of the container. Then we can take out all the leftover liquids — so it's just water with particles inside. We take out the water, and we're left with just a very dense packing of particles. And this is really all it takes to get the structural color.
AFP: This is how Victoria and Annie make blue and green pebbles of different hues. Some are shiny, like peacock feathers – but others are matte, and they have no iridescence at all. And it turns out that making structural color look matte, like the cotinga bird feather, or like the blue on a blue jay’s wing, is actually really tricky. A lot of structural colors in nature are iridescent.
VH: So if you zoom in to one of these iridescent structural color samples, then what you would see is a crystalline arrangement. And that means that on the nano scale, the particles are arranged in a very ordered way, in an ordered lattice. If you rotate the sample then the light will interact in a different way.
AFP: Imagine this orderly pattern of particles is a stack of dice or kids’ blocks, one on top of the other. If you look at it from the side, you see the square faces of the blocks. But if you look at it from an angle, you just see the edges of the blocks. Or, you could look at it from above, and only see the one face that’s pointed up toward the ceiling. When light passes through a well-ordered structure, like your tower of blocks, you get all of these angle-dependent colorful effects. And angle dependence is what gives us those shiny, iridescent colors. But let’s say you don’t want the color to be angle-dependent.
VH: If you have a very disordered material — in our lab we call them amorphous or glasses, in that case the particles are arranged in a very disordered manner, so no matter how you rotate the sample the light will always see the same material, essentially, it will see the same internal structure.
AFP: So if, instead of a tower, you threw your blocks into a clear plastic bag, the jumble of blocks wouldn’t look particularly different from one angle or another. In a disordered structure, the angle-dependent color effects disappear. Victoria and Annie brought a sample from their lab that shows order and disorder in one little vial.
AS: Yeah, we can start with that one if you want.
AFP: Yeah. This one — this one kind of blows my mind.
AS: So when you look at the sample you can see that when you hold it still, it starts to look iridescent. You see these bright colors at certain angles and when you change the angle that you look at it you see a slightly different color. And then when you shake it up, you can see that the color disappears while you're shaking it. But the second that you stop shaking it, the iridescence comes back, the particles settle back into the positions that allow you to see that iridescent color. And the reason why you're seeing this iridescent color in this liquid form is because those particles are very highly charged. And when the particles are very highly charged, they all want to get far away from each other as possible. So they're all arranging into this position that allows them to be equally far away from one another, and it creates this crystal structure- this highly ordered structure which is exactly the structure that we talk about is required to make an iridescent structural color. But when you are shaking it up, you're disrupting that crystal structure that is settled into, and you're breaking it up, and making it completely disordered, so you just get this white-looking scattering color.
AFP: It really just looks like some sort of unicorn, like in a bottle.
AS: Unicorn blood.
AFP: Unicorn blood, right yeah.
AS: It's everyone's favorite.
AFP: A former postdoc made the unicorn blood sample, and it was basically just for fun – just to test the boundaries of what structural color made in a lab could be. But structural color has some very practical applications.
AS: There is a huge range of different applications for structural color. So the first most obvious one is just that any any place that you use color currently you could substitute structural color. So, in paints, in dyes, in car coatings, all of these areas you could use structural color instead.
AFP: There are a couple big advantages to using structural color instead of regular dyes: The colors won’t fade from light exposure. And, perhaps even more interestingly, they can be made from non-toxic substances, even edible substances.
AS: When you want to make a dye now ,you're limited to this very small number of different chemicals to make that certain color. And if you want to make something that you're going to maybe eat, then you don't necessarily want to be putting all those chemicals in your food. But with structural color, you might be able to make something out of some kind of food-based or safe-to-eat material that wouldn't be so bad to put in your food.
VH: And another application for structural color is makeup, actually, and there are already several companies that are marketing this.
AFP: Not only would this makeup be less toxic, but it would also allow people to play with color and iridescence in a whole new way. So, after all this scientific research, we’re getting ready to use structural color pretty much exactly the way birds use it – to make ourselves look interesting!
VH: So when you look at a person wearing this makeup, and you look at that the skin from different angles, it has different tones, or different colors.
AS: And you can makeup that just looks more interesting.
AS: Having these iridescent colors is something that you can't really get with dyes that we currently have. So you can imagine having these like peacock-feather-looking eyes or something.
AFP: There are also potential applications in fabric, so that clothing could change its color as it moves or stretches. Even though structural color has some really powerful practical applications, it’s also important to understanding how color works at a fundamental level.
AS: We’re not just making something for an application. It's not like we're just trying to push forward in and make something to sell it. We're really trying to understand fundamentally how light scatters these materials, because it's really not understood. It's still kind of an open question.
AFP: We human beings aren’t just satisfied to look at beauty in nature. We want to take it apart, figure out how it works – and then make something that serves our needs.
AS: So I think — I think scientists know that nature does a very good job at making things and that we can take some inspiration from nature to try to make things in a lab.
AFP: People have been trying to replicate these complex, beautiful colors for hundreds, if not thousands of years. Even with limited technology, people could crush up beetles or flowers to make red or yellow dyes. But when you crush up something blue, something that has a structural color, it won’t work as a dye for clothing or other uses. The desire to re-create nature’s most impressive plumage has led to all sorts of advances in science and technology.
VH: It's well known that, historically, making blue dyes was very complex very hard for a very long time, until the chemistry got a lot more advanced and people were able to manufacture these in a lab. But blue dyes were really hard to get from nature. And understanding why nature instead chose to go with blue structural color – that’s also quite interesting, because blue dyes are so hard to make, nature instead chose to make blue structural color. Or you can think of it the other way around, because red structural color is hard to make. All the red dyes that you've seen nature come from pigments or dyes. I think that understanding how nature adapts to the circumstances — that's also quite interesting. It makes you appreciate the world.
AFP: Applied physics can teach us how to use technology to manipulate light and create plumage-inspired color that people can use for makeup or clothing or whatever else they like. And evolutionary biology can teach us how birds use plumage to communicate to potential mates. But there’s an aspect of plumage that we haven’t talked about – and that’s how human beings use plumage to communicate with each other. What signals do we send when we decorate ourselves with clothing, jewelry, and makeup? And how has that changed over time?
CHLOE CHAPIN: There's a belief within fashion studies that fashion started in the Burgundian courts in the 15th century but there's also an argument that the very first examples of decorations of humans that we have from Venus figures that include decorative hair braiding and string skirts are also a kind of fashion.
AFP: Next week on Veritalk: Episode 3, Puttin’ on the Ritz.