Bio‐inspired Highly Scattering Networks via Polymer Phase Separation

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structure factor) and their specific characteristics (form factor and refractive index). [1][2][3][4] Therefore, most efforts to control light propagation in random systems have been focused upon the optimization of isotropic spatial correlations in highrefractive index systems. These studies had a remarkable impact in many fundamental [5][6][7][8][9][10][11][12][13][14][15][16] and applied phenomena. [17][18][19][20][21] However, the role of anisotropy in multiple scattering systems has been overlooked. In fact, despite the well-known analytical solution for the single scattering, [4,22] the response of an ensemble of randomly arranged anisotropic particles has not been theoretically investigated. The same is valid for the presence of anisotropic structural correlations. Similarly, recent experimental works focused on the fabrication and optical characterization of anisotropic, disordered materials but without proving the role of anisotropy in scattering optimization. [23][24][25][26][27][28][29][30] Here, we study the effect of structural and single-particle anisotropy on the opacity of a material and identify the criteria to improve scattering over a large parameter space, including filling fraction and refractive index. Our work proves that ensembles of uncorrelated, anisotropic particles outperform their isotropic counterpart both for low-and high-refractive indices. In addition, anisotropic, low-refractive index systems not only require a lower amount of material to maximize scattering than isotropic media, but they also exhibit a whiteness comparable to those of high-refractive index materials.Our results showcase how to exploit natural resources (e.g., biopolymers) to replace commercially available white enhancers, made from inorganic high-refractive materials (e.g., TiO 2 ), which have recently raised safety concerns. [31,32] Moreover, our work unveils why anisotropy is an evolutionary-chosen mechanism to optimize scattering in biological systems. Results and DiscussionTo establish a comparison between different disordered materials, we numerically investigated the optical properties of 2D media (see Sections S1 and S2 in the Supporting Information). Exploiting a 2D numerical approach allows to independently vary the structural and form factors of a system, and therefore to disentangle their role in scattering optimization, while avoiding computational burden. In particular, we systematically The ability to manipulate light-matter interaction to tailor the scattering properties of materials is crucial to many aspects of everyday life, from paints to lighting, and to many fundamental concepts in disordered photonics. Light transport and scattering in a granular disordered medium are dictated by the spatial distribution (structure factor) and the scattering properties (form factor and refractive index) of its building blocks. As yet, however, the importance of anisotropy in such systems has not been considered. Here, a systematic numerical survey that disentangles and quantifies the role of different kinds...

Section: Introductionmentioning

confidence: 99%

Role of Anisotropy and Refractive Index in Scattering and Whiteness Optimization

Jacucci

Bertolotti

Vignolini

2019

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“…Through evolution over millions of years, elaborate biophotonic microstructures existing in creatures have been fascinating tools to efficiently control the light propagation, leading to tremendous advances in design of modern biomimetic materials and devices . Structural whiteness (i.e., broadband reflection) is one of the intriguing phenomena, which prevalently appears in insects, aquatic organisms, mammals, and avifaunae, providing ponderable ideas for applications ranging from displays to energy‐efficient thermal controlled materials and devices . Derived from the correlation between intensity of reflection light and viewing or incident angles, structural whiteness can be divided into two types, diffuse whiteness (angle independent) and metallic silver (angle dependent).…”

Section: Introductionmentioning

confidence: 99%

Bright Silver Brilliancy from Irregular Microstructures in Butterfly Curetis acuta Moore

Liu

Wang

Yang

et al. 2019

Structural whiteness in nature is one of the useful optical phenomena contributed by refined microstructures and serves important biological functions. However, whiteness residing in wings, antennae, and bodies of butterflies often draws less attention, with undetected physical mechanisms and applications. Here, it is discovered that bright silver scales on the ventral side of butterfly Curetis acuta Moore have enhanced broadband reflection with particular angle dependency, which is dominated by the irregularity of scales. The broadband reflection is the result of color mixing effect caused by laminated scales with irregular layer space and thickness. The special angle dependency suggests that the reflection intensity changes with azimuth angles but remains unvaried at different viewing angles along given direction. This is the consequence of diffraction caused by ridges and irregular holes on the scales, which generates the shining effect and extends the viewing angle of broadband reflection. The characteristic reflection property is speculated to be used for intraspecific communication. Furthermore, it is found that the broadband reflection property of the silver scales is helpful to lower the body temperature under direct sunshine. These findings for light and thermo controlling would offer a potential strategy to design bioinspired advanced materials for low‐energy, reflectance‐based applications.

“…Nonetheless, we note that our compenetrating BRW algorithm is also designed to form an interconnected phase through the unit cell to generate self‐sustaining structures. Moreover, we note that previous optimization claims are recently being further questioned by the demonstration of simple techniques allowing to fabricate similar network‐like polymer structures with scattering properties that significantly outperform those of the Cyphochilus beetle …”

mentioning

confidence: 97%

Optimized White Reflectance in Photonic‐Network Structures

Utel

Cortese

Wiersma

et al. 2019

freedom to tune light transport properties in these media. Indeed, shapes such as prolate ellipsoids or cylinders can be packed up to higher densities delaying the onset of spatial correlations at the cost of increased angular correlations. [10][11][12] Interestingly, both these aspects-namely, the high density and prevalent orientation exhibited by packed rods-can contribute to increase the overall turbidity, which makes cylinders particularly suited to realize highly turbid materials with a flat response over a broadband wavelength range. [13] Indeed, the smallest possible transport mean free path for a given refractive index contrast reported so far in the visible range has been obtained with GaP nanocylinders [14] and, for lower refractive index materials, claims of optimized scattering exist for the chitinous network structure of the Cyphochilus beetles. [5,13] In recent years, the latter has inspired an array of bio-mimicking materials attempting to reproduce its outstanding efficiency in terms of strong light scattering and limited material usage, taking advantage of a wide range of fabrication techniques including electro-spinning, super critical CO 2 foaming, polymer phase separation, and direct laser writing, [6,7,9,[15][16][17] to name a few.However, as opposed to nanoparticle systems, network materials are characterized by several additional aspects other than number density and spatial correlations, which makes it difficult to understand what key parameters should be optimized to design highly scattering network structures.Few notable examples include phase percolation, angular correlation, and network valence, all of which concur in determining their scattering properties. [13,[18][19][20] In this respect, simple generative models for photonic structures allowing to investigate the effects of these parameters separately are much needed to gain insight on their role and relevance.In this work, we describe a simple branching random walk (BRW) algorithm to generate random network structures inspired by that of the Cyphochilus beetle. Notably, the model allows to control and vary independently the volume fraction and degree of angular correlations without altering structural parameters such as the slab thickness, the shape, and aspect ratio of the constituent elements. The optical and transport properties of the generated structures have been investigated through finite difference time domain (FDTD) calculations and an inverse Monte Carlo (MC) approach, showing that the bright reflectance of the Cyphochilus beetle can be easily matched and surpassed by acting solely on the degree of anisotropy and the volume fraction of the network. To the best of our knowledge, this is the first rigorous demonstration of the key role played by structural anisotropy in highly reflective disordered samples.3D disordered networks are receiving increasing attention as they represent a versatile architecture for highly scattering materials. However, due to their complex morphology, little is known about the interplay betwee...