Bioinspired materials 2 Flashcards
When have air, thin layer of oil on surface with n1n3 respectively. What optical path length is needed for constructive/destructive interference? Normal incidence.
At the first interface is the wave phase shifted by lambda/2, since it is reflected from thinner to denser medium. At the second interface, there is no phase shift but the distance traveled extra is 2t, thickness.
Which means that this difference 2t - lambda/2 should be either mlambda for constructive interference or (m+1/2)lambda for destructive (m integer).
And lambda = lambda_0/n, lambda_0 is wavelength in air.
Which gives for constructive: n*t = lambda_0/4 (m=0)
Destructive: n*t = lambda_0/2 (m=0)
If have a system of air, some coating with n2=1.3 on glass, this means have n1
Now have phase shifts at both interfaces so these will cancel out only have the difference in path traveled, that is 2t. So have either 2t=mlambda for constructive or 2t=(m+1/2)*lambda.
Which gives for constructive: nt = m/2lambda_0
Destructive: n*t = lambda_0/4 (m=0)
Explain the principle of having multiple reflectors.
One layer only reflects a few % but many layers together reflect almost all light. Can also have different thickness of the layers to reflect different wavelengths.
How much light that is reflected at an interface depends on the difference in refractive index between the two media.
R = (n1^k-n2^k)^2/(n1^k+n2^k)
What decides the width of the reflectance peak in the spectrum?
If infinitely many layers can have perfectly thin peak, but if have large difference in refractive index delta n = n2-n1, larger bandwidth is reflected, ie. the peak is wider.
To have a narrow peak one should have a small delta n, and have many layers.
Explain structural color opposed to pigment color.
When have structural color use constructive interference of some wavelengths and destructive of some to enhance some wavelengths and give color.
Can also have changing color depending on the incident angle, can look like different color from different angles.
When angle increase –> wavelength decrease.
Give examples of beatles using structural color.
Ex. 1: One beatle only reflect green light because all layers are optimized to reflect this wavelength, but another have different thickness of the layers to reflect all wavelengths and it looks golden.
Ex. 2: A beatle combining structural and pigment color. It has ordered layers that reflects a yellow color, but when one touch the beatle the order of these layers are destroyed and the light goes through down to the red pigment molecules and it looks red instead.
Explain reflecting camouflage of fish.
When a fish is swimming with its body vertical, it does not reflect the light coming from above the water surface because it is parallel, but when it is slightly tilted it does reflect all colors because it has three different tails, and it has a silver color.
Give an example on how to use multilayer mirrors in an experiment.
When doing some fluorescence measurement, the sample can be placed on multilayered mirrors to get more intensity from the fluorescence. For example, when one wants to see if a DNA is present in a solution, if it binds to the complementary DNA on the surface it will be illuminated and the dye will fluorophore.
Explain how synthetic silk can be fabricated and analysed.
If having a film of synthetic silk and treat it with methanol the surface will become more rough and most of the alpha-helices will transform to beta-sheets, and these will form nano-crystals. Can use GIRXD (grazing incidence X-ray diffraction) to study the fraction of beta-sheets to alpha-helices.
