Precious opals owe their striking appearance to a structural periodicity that establishes arrays of interstitial voids, acting as a natural diffraction grating for light. The same periodicity makes self-assembling colloidal structures potentially useful in technologies that rely on enhanced optical transmission. But establishing long-range stability in these synthetic materials has so far proved challenging. Now, Flavio Romano and Francesco Sciortino have come up with a way of stabilizing self-assembled colloidal structures over long length scales.
Particles in these systems effectively interact only with their nearest neighbours, because the potentials act over distances comparable to, or smaller than, the size of the particles themselves. This means that colloidal systems tend to form hybrids comprising crystalline structures that share similar local configurations — such as face-centred-cubic and hexagonal-close-packed lattices.
By engineering colloids patterned with attractive spots on their surface — so-called patchy particles — Romano and Sciortino were able to overcome these difficulties. Their technique, specifically tailored to pattern particles asymmetrically, succeeds in suppressing hexagonal structures, in favour of a pure crystalline state. The strategy is even robust to imperfections in patch placement that may arise from the fabrication process.
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Klopper, A. Patch work. Nature Phys 8, 637 (2012). https://doi.org/10.1038/nphys2418
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DOI: https://doi.org/10.1038/nphys2418
