RESEARCH

I am broadly interested in polymers, colloids, self-assembly, optics, and special biomaterials (i.e. melanin) from both fundamental and practical aspects. Fundamental questions include (1) investigating the mechanism of color production in bird feathers; (2) optimizing structural colors based on both bio-inspiration and optical modeling; (3) understanding the mechanism of self-assembly of nanoparticles at the interface; (4) exploring the physics of multiple scattering in disordered photonic materials. Practical challenges include (1) creating bio-inspired melanin-based photonic structures for applications like sensors, coatings, or inks; (2) developing new self-assembly process to fabricate structurally colored materials; (3) engineering wide-angle camouflage and displays. Through my previous research trainings (Figure 1), I have filled my interdisciplinary toolbox that contains knowledge of polymer chemistry and physics, expertise in colloids and optics, skills in dealing with biological photonic systems. I have been actively collaborating with both academic groups and industry partners, helping me understand needs and challenges in different research fields and real applications.

1. Surface segregation in photonic colloidal assemblies
Science Advances, 2019, 5, eaax1254
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Surface segregation in binary colloidal mixtures offers a simple way to control both surface and bulk properties without affecting their bulk composition. Here, we combine experiments and coarse-grained molecular dynamics (CG-MD) simulations to delineate the effects of particle chemistry and size on surface segregation in photonic colloidal assemblies from binary mixtures of melanin and silica particles of size ratio (Dlarge/Dsmall) ranging from 1.0 to ~2.2. We find that melanin and/or smaller particles segregate at the surface of micrometer-sized colloidal assemblies (supraballs) prepared by an emulsion process. Conversely, no such surface segregation occurs in films prepared by evaporative assembly. CG-MD simulations explain the experimental observations by showing that particles with the larger contact angle (melanin) are enriched at the supraball surface regardless of the relative strength of particle-interface interactions, a result with implications for the broad understanding and design of colloidal particle assemblies.
 
2. Bioinspired bright noniridescent photonic melanin supraballs
Science Advances, 2017, 3, e1701151.
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Structural colors produced so far lack the brightness and saturation at wide viewing angles; and manufacturing structural colors has been challenging. In this work, we use a simple and scalable one-pot reverse emulsion process to assemble melanin and silica core-shell nanoparticles to produce bright and non-iridescent photonic inks (micron-size supraballs). High brightness and saturation of the colors result from the careful design of the core-shell nanoparticle and broad band absorption of melanin; and the non-iridescent effect is due to the spherical symmetry of supraballs.
 
3. Bio-Inspired structural colors produced via self-assembly of synthetic melanin nanoparticles
ACS Nano, 2015, 9, 5454.
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Inspired by the wide use of melanin in bird colorations, we are pioneered to synthesize mondodispersed melanin nanoparticles and assemble them into structurally colored films. We are first to quantify the optical constant of synthetic melanin nanoparticles and demonstrate that a high refractive index and broad absorption leads to high saturation in assembled melanin films. The significance of this work can also been seen from the fact that this work was one of the 20 most read papers in the journal that year, and it has been downloaded ~ 19 000 times and drawn extensive highlights from different sources like popular science.
 
4. Humidity colorimetric sensors based on bio-inspired synthetic melanin nanoparticles
Chemistry of Material, 2016, 28, 5516.
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In this work, we demonstrate that bio-inspired films composed of self-assembled synthetic melanin nanoparticles (SMNP) exhibit fast (~seconds), distinctive (blue to green, red to green), and reversible color changes under varying humidity. We systematically investigate the origins of the colors change upon humidity variations, which provides the insights on the design of practical naked-eye colorimetric sensors. More importantly, humidity-induced dynamic colors arising from SMNP films offer possible routes for synthetic melanin as an important material in sensors and coatings.
 
5. Elucidation of the hierarchical structure of natural eumelanins
Journal of The Royal Society Interface, 2018, 15, 20180045.
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Eumelanin is one of the most ubiquitous pigments and its chemical structure remains unclear. In this study, we use atomic force microscopy, X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (NMR) to compare intact pure eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin
 
6. Nanostructural basis of rainbow-like iridescence in Australian pigeon feathers
Optics Express, 2014, 22, 14625.
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In this work, we demonstrate the full spectrum of colors (from blue to red) in a single piece of common bronzewing feathers is produced through subtle shifts in both spacing and diameter of melanosomes in a multilayer structure. Although spacing differences in multilayers are common, subtle variation in melanosome size has never before been reported. This finding illustrates tight developmental control in feathers and provides inspiration for the design of multi-colored coatings or fibers.