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3 Broken Symmetry
Better ordered structures of this kind are called colloidal crystals. They may
have the same structure, most commonly the densest, that is, face- or body-centered
cubic as in an atomic crystal, but consist of particles of a colloidal size. A natural
mineral of this kind is opal, a close-packed array of silica spheres. What makes opal
a gemstone is that the internal scale of a colloidal crystal is of the same order of
magnitude as the wavelength of visible light. Just as X-rays are diffracted by atomic
crystals, thereby revealing their structure, so is light when it shines on opal, giving
it either an opalescent appearance or brilliant colors.
Changes in lattice spacing affect the transparency and reflectivity with respect to
light at a certain wavelength, which earns them yet another appellation – photonic
crystals. Animals took advantage of this before us. Chameleons change their color,
either for camouflage or for social signaling, by modifying the lattice spacing of
a nanocrystalline layer that covers their skin, as in the left-hand panel of Fig. 3.6
(Milinkovitch, 2015). Some butterflies owe their brilliant colors to similar layering
on their wings. They are not smart enough to manipulate it, but another insect, the
Hercules beetle, can do this (Hinton and Jarman, 1972). The transparent cover of
its wings is underpinned by a spongy layer containing an array of pillars normal to
the surface (Fig. 3.6, right). The beetle may fill this layer either with air to make it
yellowish or with water to turn it black.
In technology, the Hercules beetle’s trick is used in optical sensors. Here the
aim is not just to change color but to investigate the properties of a fluid filling
the porous space of a mesoscopic crystal. The structure, called a reverse opal, is
prepared by first assembling monodisperse silica or polymer spheres in a closepacked lattice, and then filling the interstices with another material and removing
those spheres to create an ordered porous medium. The void can be filled by a fluid
Fig. 3.6 Left: Change in the lattice spacing (top) and the resulting color change of a chameleon
in a relaxed and excited state (bottom). Right: Side view of an array of pillars in Hercules beetle
wings, cut in the middle to show the top view
3 Broken Symmetry
Better ordered structures of this kind are called colloidal crystals. They may
have the same structure, most commonly the densest, that is, face- or body-centered
cubic as in an atomic crystal, but consist of particles of a colloidal size. A natural
mineral of this kind is opal, a close-packed array of silica spheres. What makes opal
a gemstone is that the internal scale of a colloidal crystal is of the same order of
magnitude as the wavelength of visible light. Just as X-rays are diffracted by atomic
crystals, thereby revealing their structure, so is light when it shines on opal, giving
it either an opalescent appearance or brilliant colors.
Changes in lattice spacing affect the transparency and reflectivity with respect to
light at a certain wavelength, which earns them yet another appellation – photonic
crystals. Animals took advantage of this before us. Chameleons change their color,
either for camouflage or for social signaling, by modifying the lattice spacing of
a nanocrystalline layer that covers their skin, as in the left-hand panel of Fig. 3.6
(Milinkovitch, 2015). Some butterflies owe their brilliant colors to similar layering
on their wings. They are not smart enough to manipulate it, but another insect, the
Hercules beetle, can do this (Hinton and Jarman, 1972). The transparent cover of
its wings is underpinned by a spongy layer containing an array of pillars normal to
the surface (Fig. 3.6, right). The beetle may fill this layer either with air to make it
yellowish or with water to turn it black.
In technology, the Hercules beetle’s trick is used in optical sensors. Here the
aim is not just to change color but to investigate the properties of a fluid filling
the porous space of a mesoscopic crystal. The structure, called a reverse opal, is
prepared by first assembling monodisperse silica or polymer spheres in a closepacked lattice, and then filling the interstices with another material and removing
those spheres to create an ordered porous medium. The void can be filled by a fluid
Fig. 3.6 Left: Change in the lattice spacing (top) and the resulting color change of a chameleon
in a relaxed and excited state (bottom). Right: Side view of an array of pillars in Hercules beetle
wings, cut in the middle to show the top view