What birds have taught us about structural colors
School of Engineering and Applied Sciences
Department of Physics,
Harvard University
Funding: International Collaboration grant (No.Sunjin-2010-002) from Korean Ministry of Knowledge Economy, Harvard MRSEC (NSF DMR-0820484)
Sofia Magkiriadou, Jin-Gyu Park, Shin-Hyun Kim (KAIST), Young-Seok Kim (KETI), Gi-Ra Yi (SKKU),
Adeline Perro, Guangnan Meng,
Vinothan N. Manoharan
Dramatis Personae
Sofia Magkiriadou Jin-Gyu Park
Young-Seok Kim (KETI)
Shin-Hyun Kim (KAIST)
Jason Forster (Yale)
Gi-Ra Yi (Sungkyunkwan U)
Eric Dufresne (Yale)
(collaborating with Mochrie and Prum)
Ordinary color and structural color
transmitted red and green
incident white light
scattered blue light
absorption of red and green
broadband adsorption
constructive interference of
scattered blue light
Physical models of structural color
Multilayer interference (Rayleigh, 1917)
l
Incident wave Scattered waves
Diffraction (Bragg & Bragg, 1913)
Bragg’s Law:
Bragg diffraction: a simple model for iridescence in porous structures
2 sinn dl
l
Incident wave Scattered waves
5 years ago: biomimetic structural color
Forster, Noh, Liew, Saranathan, Schreck, Yang, Park, Prum, Mochrie, O’Hern, Cao, Dufresne Advanced Materials 22: 2939 (2010). See also: García, Riccardo, and López. Advanced Materials (2010)
A C
0.5 μm 2 μm 18
cm
Dufresne (Yale)
Dried polystyrene microspheres (~250 nm)
Bragg’s Law:
In ordered materials, the color depends on viewing angle
2 sinn dl
~5°with respect to light source
~50°
1 μm 1 μm
l
Incident wave Scattered waves
In amorphous materials, the color is angle-independent
Same sample, tilted by ~ 60 degrees (SM).
226 nm polystyrene beads
3 m
m
Angle-independent color also comes from constructive interference
Short-range correlations between particles → Constructive interference
Rotational symmetry → Orientation independence
Str
uct
ure
fac
tor
S(q
)
q
θ Forster et al., Adv. Mater. 2010
correlations with wavevector
𝑞structure ≈ 2𝜋/𝑑
interparticle distance d
𝑞scat = 4𝜋/𝜆 sin 𝜃/2
𝑞scat = 𝑞structure at resonance
wavelength of structural color: 𝜆color ≈ 2𝑑
Consider reflected light: 𝜃 = 𝜋
An application: reflective color displays
w/ Young-Seok Kim (KETI) Gi-Ra Yi (SKKU) Shin-Hyun Kim (KAIST)
A proposal: make color “ink” particles for flexible, reflective color displays (Grant No.Sunjin-2010-002 from Korean Ministry of Knowledge Economy)
Requirements:
- wide viewing angle
- small particles (~10 micron)
- saturated color
Potential problems for display applications
Pictures from Forster et al., Adv. Mater. 2010
White color except in thin films (or with absorbing materials added)
Appearance (saturation, transparency) not easily controlled
How to make “ink” particles?
Analysis: length scales
scaQ
al
l
3
4
)( 1
cos
)1/(*
g
gll
Forster et al., Adv.Mater. 2010
Sample thickness L (or absorption length Labs)
Scattering length l distance over which
intensity decays
Transport length l* distance over which
propagation direction is randomized
L
g: asymmetry parameter g = 0 : isotropic g = 1 : forward scattering
Qsca: scattering efficiency
Design rule: minimize multiple scattering (maximize transport length) of non-resonant wavelengths
incident white light
scattered blue light
transmitted light
Design rules:
L, Labs < l* (no multiple scattering)
2d ~ λ (resonance condition)
Problem: transport length depends on particle size, which also determines color
20-200 nm
200 - 400 nm
Solution: make composite particles
(decouple interparticle spacing from particle size)
Dense core
Sparse, low refractive index shell
Synthesis of core-shell particles
Perro, Meng, Fung, Manoharan, Langmuir, 25(19), 11295-11298 (2009)
PS nanoparticles PS-poly(NIPAM-co-AAc) core-shell particles
Seeded emulsion polymerization
Making a low-refractive index shell: add acrylic acid to make shell sparse
Perro, Meng, Fung, Manoharan Langmuir 2009
PS cores
(Shells are invisible)
Optical micrograph after drying
Core-shell particles allow resonant wavelength to be tuned independently of scattering strength
Magkiriadou, Sofia, Jin-Gyu Park, Young-Seok Kim, and Vinothan N. Manoharan. 2012. Optical Materials Express 2 (10): 1343.
core: 180 nm
shell < 430 nm
core: 180 nm
shell < 640 nm
But there are limitations
Images of samples made with different core and shell diameters
diameter = 170 nm
An experiment: solid PMMA particle glasses
240 nm 330 nm
No red? 𝜆color ≈ 2𝑑
Why does red fail?
New peak appears in the blue
diameter = 170 nm
PMMA particle glasses
diameter = 240 nm diameter = 330 nm
Particles from R.Guerra, Weitz group
Detected intensity is related to the form factor and the structure factor:
𝐼detected ~1
𝑘2 𝐹 𝑞 𝑆 𝑞 𝑞 𝑑𝑞
𝐹 𝑞 = 1
𝑘2𝑑𝜎
𝑑Ω
𝑆 𝑞 =1
𝑁 𝑒−𝑖𝐪∙𝐫𝑗
2
Consider only backscattering (𝑞 =4𝜋𝑛eff
𝜆): 𝑆 𝜆 , 𝐹(𝜆)
Single-scattering model
Magkiriadou, Park, Kim, Manoharan, Physical Review E 2014
Form factor: differential scattering cross-section of a single particle
Structure factor: interference between scattered waves
q
θ
Two peaks: one from structure S(q), one from individual particles F(q)
Structure factor S(q)
(from Percus-Yevick relation)
Form factor F(q)
(from Mie theory)
Combined F(q)S(q)
Magkiriadou, Park, Kim, Manoharan, Physical Review E 2014
Backscattering resonances from single particles are responsible for blue peak
Magkiriadou, Park, Kim, Manoharan, Physical Review E 2014
Resonances in F(λ) occur at approximately λ = 2 n d / m, n: refractive index of the particle m: integer
Not really a Mie resonance (which are in the UV)
Can we minimize the blue peak?
resonances occur at 𝜆 ≈ 2𝑛particle𝑑/𝑚
F(q): single-particle scattering resonances S(q): Interference effects
resonances occur at 𝜆 ≈ 1.7𝑛medium𝑑
d
Optimize 𝑛particle, 𝑑core, 𝑛medium to shift these two resonances
water
oil
Aqueous suspension of core-shell particles enclosed in a droplet
• δ: core size (polystyrene) • d: shell size (PNIPAM-co-AAC) • L: capsule size
How to shift the resonances independently: photonic capsules of core-shell particles
L
Photonic capsules with microfluidics
S. H. Kim, J.-G. Park, T. M. Choi, V. N. Manoharan, and D. A. Weitz, Nature Communications 2014. J.G. Park, S.-H. Kim, S. Magkiriadou, T. M. Choi, Y.-S. Kim, V. N. Manoharan, Angewandte Chemie International Edition 2014
1) Microfluidics generates 100 μm double emulsions containing 200-300 nm core-shell particles
2) Controlled shrinkage packs particles and generates tunable structural color
A. (9/1. #/#)
B.
+ A. B. C.
Tuning color by changing indices and diameters
“Photonic pigments” with isotropic structural color
J.G. Park, S.-H. Kim, S. Magkiriadou, T. M. Choi, Y.-S. Kim, V. N. Manoharan, Angewandte Chemie International Edition 2014
Structural colors are isotropic and span visible spectrum
Optimizing the color: what’s the best red you can get?
Magkiriadou, Park, Kim, Manoharan, Physical Review E 2014
Shift blue peak in F(q) to UV by making 𝑛𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 < 𝑛𝑚𝑒𝑑𝑖𝑢𝑚
S(q)
F(q)
F(q)S(q)
Does it work?
Shift blue peak in F(q) to UV by making 𝑛𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 < 𝑛𝑚𝑒𝑑𝑖𝑢𝑚
PMMA / silica Immersion oil (n ~ 1.47 – 1.70)
silica infiltrated with Cargille oil A 1.49
silica 250 nm dry
Conclusion
Bird-like structural color is a consequence of short-range correlations and well-chosen refractive index contrasts,
microstructures
Collaborators: Gi-Ra Yi (SKKU), Young-Seok Kim (KETI), Shin-Hyun Kim (KAIST)
Thanks to:
Jason Forster, Eric Dufresne, Richard Prum (Yale University)
Shin-Hyun Kim (KAIST), Rodrigo Guerra, David Weitz (Harvard)
Ariel Amir (Harvard)
Aspen Center for Physics and NSF PHY-1066293
Funding:
International Collaboration grant (No. Sunjin-2010-002) from Korean Ministry of Knowledge Economy
Harvard MRSEC (National Science Foundation DMR-0820484)
People and Acknowledgements
Sofia Magkiriadou Jin-Gyu Park