This Season's Colours

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one critical bundle radius, derived from the the assemblies can be tuned (it should be e-mail: [email protected]; relative energetic costs of filament bending noted that the models are idealized and [email protected] and interfilament spacing distortion. Below do not contain all the relevant elements; this critical radius, the preferred morphology particularly, entropic and thermal effects References corresponds to bundles with a circular are not yet included, and could turn out 1. King, H., Schroll, R. D., Davidovitch, B. & Menon, N. Proc. Natl Acad. Sci. USA 109, 9716–9720 (2012). cross-section, whereas above it ribbon-like to be important). The task now is to find 2. Irvine, W. T. M., Vitelli, V. & Chaikin, P. M. Nature assemblies are favoured. Experimental experimental ways of controlling and 468, 947–951 (2010). and numerical verification of the authors’ manipulating these physical parameters, 3. Hure, J., Roman, B. & Bico, J. Phys. Rev. Lett. 106, 174301 (2011). 4. Meng, G., Paulose, J., Nelson, D. R. & Manoharan, V. N. Science predictions provides reinforcement of this for example via variation of temperature, 343, 634–637 (2014). simple yet insightful theory. solvents and concentrations. ❐ 5. Hall, D. M., Bruss, I. R., Barone, J. R. & Grason, G. M. Grason and colleagues’ modelling study Nature Mater. 15, 727–732 (2016). 6. Chiti, F. & Dobson, C. M. Annu. Rev. Biochem. is an important step towards the quantitative Eran Sharon is at the Racah Institute of Physics, 75, 333–366 (2006). understanding — and eventually, better The Hebrew University of Jerusalem, Jerusalem, 7. Seung, H. S. & Nelson, D. R. Phys. Rev. A 38, 1005–1018 (1988). control — of physical and chemical Israel. Hillel Aharoni is in the Department of 8. Schneider, S. & Gompper, G. Europhys. Lett. 70, 136–142 (2005). 9. Bruss, I. R. & Grason, G. M. Proc. Natl Acad. Sci. USA self-assembly. It defines the physical Physics and Astronomy, University of Pennsylvania, 109, 10781–10786 (2012). ‘handles’ with which the morphology of Philadelphia, Pennsylvania 19104, USA. 10. Bruss, I. R. & Grason, G. M. Soft Matter 9, 8327–8345 (2013). MATERIAL WITNESS THIS SEASON’S COLOURS When the microscope was first material, scaled up in size so as invented in the early seventeenth to work at infrared wavelengths4. century there was a hope that it would Turner’s group has now used optical reveal mechanical origins for all of lithography to write a gyroid network nature’s properties. That, alas, was not into a photo-resin with a lattice the case. But Robert Hooke discerned constant as small as 300 nm, less that colour, at least, could result from than the C. rubi lattice of 350 nm the “curious and exceeding smallness and which reflects strongly in the and fineness” of nature’s parts. He near ultraviolet5. studied the colours of bird plumage, One can imagine nature shrugging pronouncing them “fantastical” because at this technical feat, reliant as it is on PHILIP BALL they would vanish when the feathers sophisticated photonic technology are wet. He correctly deduced that they rather than the mild self-organization convoluted nature of the interface arise “from the refractions of the light”1. of organic membranes that produces induces a variation in refractive Coloration from light scattering the butterfly structures6. Yet, wet index, and the plasmons get trapped was studied by Michael Faraday, chemical methods are sufficient for in nanoscale regions where this David Brewster and John Tyndall making synthetic structural colour surface is convex. Resonant coupling in the nineteenth century, but only too — or nearly so. Galinski et al. associated with the waveguided modes recently has it become feasible to have created a wide range of highly within the alumina layer then allows fabricate nanostructures with the saturated colours, from yellow to blue, some wavelengths to be strongly fineness and control needed to mimic in composite thin films in which the captured within the film while others nature’s structural colours. The light-scattering porous networks are are reflected. variety of these natural structures made by selective dissolution of a The result is a material that is is remarkable, stemming from what metal alloy7. They remove the reactive light and robust and which generates looks for all the world like a kind of aluminium from 300-nm-thick films of strong, tunable and non-iridescent biological precision engineering2. a platinum–yttrium–aluminium alloy, colours — suitable, Galinski et al. say, Typically they consist of complex leaving a random porous network. To for applications in the automobile or periodic arrays of scattering elements, create strong reflectivity at a specific aeronautical industries, and perhaps such as rods or plates of melanin or optical wavelength, the researchers for optical energy harvesting. ❐ chitin in feathers and butterfly wing deposit a dielectric film of alumina on scales. The ordered porous network top using sputtering, the colour being References known mathematically as a gyroid, governed by the thickness of this layer 1. Hooke, R. Micrographia Observation XXXVI (J. Martyn & J. Allestry, 1665). found to be responsible for the bright (tens of nm). 2. Vukusic, P. & Sambles, J. R. Nature 424, 852–855 (2003). green reflectivity of the Emerald- The colour-generating mechanism 3. Michielsen, K. & Stavenga, D. G. J. R. Soc. Interface patched Cattleheart (P. sesostris) and here is complex, without a known 5, 85–94 (2008). Green Hairstreak (C. rubi) butterflies, natural analogue. It involves an 4. Turner, M. D., Schröder-Turk, G. E. & Gu, M. Opt. Express 19, 10001–10008 (2011). is perhaps the most elegant and ornate interaction between light reflected 5. Gan, Z., Turner, M. D. & Gu, M. Sci. Adv. of them all3. internally in the alumina film, which 2, e1600084 (2016). Turner et al. have previously used acts as a waveguide, and excitations 6. Saranathan, V. et al. Proc. Natl Acad. Sci. USA 107, 11676–11681 (2010). photopolymerization to reproduce called surface plasmon polaritons at 7. Galinski, H. et al. Preprint at http://www.arxiv.org/ the gyroid structure in a photoresist the metal/dielectric interface. The abs/1605.03700 (2016).