The June, 2018 issue of the Journal of Chemical Education contains an article that describes a simple, yet fascinating experiment that you and your students are going to love! It involves the use of butterfly wings from the genus Morpho.1 The wings of these butterflies display the most beautiful blue color I’ve ever seen. I purchased some of these wings2 so I could observe them and also to see what happens when some methanol is dripped onto these wings:
What gives these wings their vivid blue color, and why do the wings change color when methanol is dripped onto them?
First, let’s discuss why the wings are blue. Remarkably, the stunning blue color is not due to chemical pigments, but rather from millions of nanoscopic structures called lamellae that are embedded on the wings (Figure 1). The lamellae contain several alternating regions of ridges and air gaps. The ridges and air gaps are nanoscopic in size: small enough to diffract visible light. Any color of light – except for blue – that interacts with the lamellae is cancelled out due to destructive interference. However, when blue light strikes the lamellae it is amplified via constructive interference. These effects cause the wings to appear an intensely blue color. Color that arises in due to structural elements rather than pigmentation is called structural coloration.3
Figure 1: Schematic of lamellae. (Left) a single, nanoscopic structure. (Right) Several lamellae.
The authors of the J. Chem. Educ. article note that the central wavelength of light,
λ = 2( n 1 d 1 + n 2 d 2
Where d1 and d2 are the dimensions of the air gaps and ridges, while n1 and n2 are the refractive indexes of the air gaps and ridges, respectively. Plugging in the known values for the dimensions (54 nm for ridges and 142 nm for the air gaps)1 and refractive indexes (1.56 for the ridges and 1.00 for air)1 we calculate that the scattered light is expected to be centered at 452 nm, which is blue light:
λ =2[( 1.00)( 142 nm) + ( 1.56)( 54 nm)] = 452 nm
We can also use Equation 1 to describe the color change that occurs when the wings are wetted with other substances such as methanol. When this is done the gaps between the ridges become filled with methanol instead of air, which changes the refractive index of the gaps. Using the value for the refractive index of methanol (1.326)1 rather than air in Equation 1, we obtain the result that the center of the wavelength of scattered light should be 545 nm, consistent with a greenish color:
λ = 2[( 1.326)( 142 nm) + ( 1.56)( 54 nm)] = 545 nm
After I observed the color changes that occur on these wings upon adding methanol and other alcohols, I began to wonder what would happen if the wings were soaked in liquid nitrogen. Here’s what I observed when I did so:
Taking into consideration the refractive index of liquid nitrogen (1.200)2 and using Equation 1, a central wavelength of 509 nm is obtained:
λ =2[( 1.200)( 142 nm) + ( 1.56)( 54 nm)] = 509 nm
This value harmonizes well with the brilliant green color observed when these wings are dipped in liquid nitrogen. The fact that the wings appear somewhat yellow-green in color when soaked in methanol but more blue-green when soaked in liquid nitrogen is consistent with the calculations above.
Finally, these wings are very difficult to wet with water:
This effect results from a combination of the high surface tension of water and the very large surface area the millions of lamellae impart to these wings. I like to think of surface tension as the energy required to get a liquid to “spread out” over a certain area. Therefore, because water has a high surface tension (72 mJ m-2),4 to it resists “spreading out” on surfaces – it tends to “bead up” instead. The lamellae on the Morpho butterfly wings mean there are hundreds of millions of crevices on the wing surface, which gives these wings enormous surface area. Because water has such a high surface tension and therefore does not spread out easily, it cannot penetrate all the nooks and crannies introduced by the lamellae. Thus, it cannot wet the wing. This probably comes in handy for the Morpho butterflies, because they live in rainy tropical regions.3 On the other hand, methanol (22.5 mJ m-2)4 and liquid nitrogen (8.9 mJ m-2)5 have considerably lower surface tensions, and they therefore easily wet these wings.
I have found that people young and old enjoy viewing experiments with the blue butterfly wings. In the video below you can view some of these experiments explained and explored in a bit more detail. Let me know if you try experimenting with blue Morpho butterfly wings – especially if you learn something new. Happy experimenting!
Acknowledgement: I wish to thank Bruce W. Baldwin for helpful discussion.
References
1. B. Bober, J. Ogata, V. Martinez, J. Hallinan, T. Leach, and B. Negru, Investigating Nanoscopic Structures on a Butterfly Wing To Explore Solvation and Coloration, Journal of Chemical Education, 2018 95 (6), 1004-1011.
2. I was able to purchase 50 wings for about $50 at The Butterfly Company. When handled carefully, the wings can be used several times. See: https://www.thebutterflycompany.com/product-category/butterflies/morphidae-blue-morphos-others (Accessed 1/29/19)
3. P. Vukusic and D.G Stavenga, Physical methods for investigating structural colours in biological systems, Journal of the Royal Society, January 2009.
4. G. Vazquez, E. Alvarez, and J. Navaza, Surface Tension of Alcohol Water + Water from 20 to 50 .degree.C, Journal of Chemical & Engineering Data, 1995 40 (3), 611-614.
5. Dortmund Data Bank, http://www.ddbst.com/en/EED/PCP/SFT_C1056.php (Accessed 1/29/19)
Safety
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas.
*More information about this category of NGSS can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions.
"Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas."
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
Assessment is limited to algebraic relationships and describing those relationships qualitatively.
Examples of data could include electromagnetic radiation traveling in a vacuum and glass, sound waves traveling through air and water, and seismic waves traveling through the Earth.
Students who demonstrate understanding can evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
Assessment does not include using quantum theory.
Emphasis is on how the experimental evidence supports the claim and how a theory is generally modified in light of new evidence. Examples of a phenomenon could include resonance, interference, diffraction, and photoelectric effect.
Students who demonstrate understanding can Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.*
*More information about all DCI for HS-PS4 can be found at https://www.nextgenscience.org/topic-arrangement/hswaves-and-electromagnetic-radiation.
Students who demonstrate understanding can Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
Assessment is limited to qualitative descriptions.
Emphasis is on the idea that photons associated with different frequencies of light have different energies, and the damage to living tissue from electromagnetic radiation depends on the energy of the radiation. Examples of published materials could include trade books, magazines, web resources, videos, and other passages that may reflect bias.