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