Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for manmade materials. Here, we show that a simple confined selfassembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal's curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies-potentially as more efficient mimics of structural color as it occurs in nature.self-assembly | colloids | photonic crystal | structural color | hierarchy H ierarchical design principles, i.e., the structuration of material over multiple length scales, are ubiquitously used in nature to maximize functionality from a limited choice of available components. Hierarchically structured materials often provide better performance than their unstructured counterparts and novel properties can arise solely from the multiscale structural arrangement. Examples can be found in the extreme water repellency of the lotus leaf (1); the outstanding mechanical stability and toughness of sea creatures such as sea sponges (2) and abalone shells (3); and the bright coloration found in beetles, birds, and butterflies (4, 5).To achieve the strongest visual effects, many organisms combine optical effects arising from light interacting with structured matter at different length scales (6). Structural periodicity on the scale of visible light wavelengths can result in regular optical density variations that give rise to bright, iridescent colors due to pronounced interference effects (4). At the micron scale, regular structural features act as diffraction gratings that produce vivid, rainbow coloration (7) and are used to control scatteri...

Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.

Mechanical forces in the cell’s natural environment have a crucial impact on growth, differentiation and behaviour. Few areas of biology can be understood without taking into account how both individual cells and cell networks sense and transduce physical stresses. However, the field is currently held back by the limitations of the available methods to apply physiologically relevant stress profiles on cells, particularly with sub-cellular resolution, in controlled in vitro experiments. Here we report a new type of active cell culture material that allows highly localized, directional and reversible deformation of the cell growth substrate, with control at scales ranging from the entire surface to the subcellular, and response times on the order of seconds. These capabilities are not matched by any other method, and this versatile material has the potential to bridge the performance gap between the existing single cell micro-manipulation and 2D cell sheet mechanical stimulation techniques.

Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this reverse color-order diffraction effect. The bioinspired system consists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the coupling of diffraction resulting from individual structural components and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticounterfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.bioinspired optics | gratings | biophotonics T hree-dimensional photonic crystals (1-5), materials with twodimensional micro-or nanosized periodic morphologies (6-8), and one-dimensional multilayer configurations (9) have been identified as the primary cause of structural coloration in a wide variety of nonrelated biological organisms. In contrast, surfaceconfined diffraction elements for the separation of incident light into specific colors are less abundant in nature and have only been discovered in a handful of organisms (10), including a fossil polychaete (7), sea mouse Aphrodita sp. (6), and some flowering plants (11). Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the scales of the butterfly Pierella luna (12).Inspired by this biological light manipulation strategy, we devised an artificial material morphology mimicking the butterfly's diffraction effect by creating periodic arrays of vertically oriented individual microdiffraction gratings. In addition to the butterflyinspired reverse color-order diffraction arising from each individual micrograting, the periodicity between the individual gratings causes diffraction on a different length scale, leading to complex intensity distributions in experimentally measured angularly resolved reflection spectra. An in-depth analysis of the observed diffraction phenomenon complemented by optical modeling revealed a strong dependence of the optical signature on the orientation of the gratings. Such an effect can only be seen because of the hierarchical nature of the superposed, orthogonal grating features. To further elucidate the role of the different structural components for the emerging reflection spectra, the initially vertically oriented individual microgratings were subjected to a tilt, resulting in a predictable change of the surface's optical signature.The dorsal side of the fore-and hind wings of P. luna males are dull brown in diffuse ambient illumination (Fig. 1A, Left). When exposed to directional illumination at grazing incidence, a coin-sized spot on each fore wing displays an angle-dependent color variation across the whole visible spectrum (Fig. 1A, Right). The color changes from re...

Abstract:Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, we combine a bottom-up assembly of polystyrene particles in the presence of a silica sol-gel precursor material (tetraethylorthosilicate, TEOS), which creates crack-free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, with top-down microfabrication techniques. We create micron scale hierarchical superstructures having a highly regular internal nanostructure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 with precisely controlled crystal orientation and wall profiles. The ability to combine structural order at the nano-and microscale enable us to fabricate materials with complex optical properties arising from light-matter interactions at different length scales. We design a hierarchical diffraction grating that combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micron-scale periodicity. Revised Manuscript

Abstract:Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. One such example is the biomineralized armor of the chiton Acanthopleura granulata, which incorporates an integrated sensory system that includes hundreds of eyes with aragonite-based lenses. We use optical experiments to demonstrate that these microscopic lenses are able to form images. Light scattering by the polycrystalline lenses is minimized by the use of relatively large, crystallographically-aligned grains. Multi-scale mechanical testing reveals that as the size, complexity, and functionality of the integrated sensory elements increases, the local mechanical performance of the armor decreases. However, A. granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multi-purpose system with cooptimized optical and structural functions.

We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120° to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.

Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.