Building Materials by Packing Spheres
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Building Materials by Packing Spheres
Vinothan N. Manoharan and David J. Pine Abstract An effective way to build ordered materials with micrometer- or submicrometer-sized features is to pack together monodisperse (equal-sized) colloidal particles. But most monodisperse particles in this size range are spheres, and thus one problem in building new micrometer-scale ordered materials is controlling how spheres pack. In this article, we discuss how this problem can be approached by constructing and studying packings in the few-sphere limit. Confinement of particles within containers such as micropatterned holes or spherical droplets can lead to some unexpected and diverse types of polyhedra that may become building blocks for more complex materials. The packing processes that form these polyhedra may also be a source of disorder in dense bulk suspensions. Keywords: colloids, microspheres, optical materials, packing, structure.
Introduction Monodisperse colloidal particles are natural building blocks for composite materials with micrometer-scale features, but these particles have one inherent limitation: they are nearly always spheres. Because a colloidal particle is much larger than its constituent atoms, its shape is determined by surface tension, not by its interior covalent bonds. The equilibrium and nonequilibrium behavior of colloidal suspensions is controlled primarily by the hard repulsive interaction between particles, and dense suspensions of identical hard spheres are known to form only fcc colloidal crystals1,2 or disordered, glassy packings. Despite this limitation, several new kinds of materials based on colloids have emerged in the past decade. Materials such as photonic crystals and macroporous media are made by drying or flocculating a dense suspension so that the particles touch and form a rigid, colloidal sphere packing. Since the colloidal length scale is commensurate with optical wavelengths, fcc colloidal crystallization is an inexpensive and simple way to prepare ordered materials that diffract light. Dried colloidal crystals can be used as templates3–5 to make photonic crystals that can function as optical semiconductors in a number of devices. The same templating procedures can be applied to either ordered or disordered particle packings to make macroporous materials, which can serve as flow-through catalyst supports, filters, and lightweight structural materials.6 MRS BULLETIN/FEBRUARY 2004
The properties of these materials depend sensitively on the microstructure of the colloidal sphere packing. In photonic crystals, grain boundaries formed during crystal growth and cracks formed during drying can incoherently scatter light, reducing the diffraction efficiency, whereas other types of defects such as vacancies can act as resonant cavities for specific frequencies, allowing us to create localized states in the optical spectrum. Unfortunately, there is no simple way to control the number and type of defects when making photonic crystals by packing spheres, nor is there a way to control the mos
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