Colloidal suspensions have been part of the repertoire of human industry since Paleolithic artists first dispersed pulverized minerals in rendered animal fat to make paint. Remarkably, colloidal suspensions' primary industrial applications have changed little in the 20 or 30 millennia since. People use colloidal suspensions to disperse materials into fluid media without dissolving them. Familiar examples include surface coatings such as paints and inks, and slurries used for paper making and powder transport. Dividing the material finely enough to disperse it sometimes instills the resulting particles with novel properties. Ancient Romans took advantage of quantum confinement effects in metallic and semiconductor nanoclusters to color glass—research on the mechanisms and applications of nanoclusters' properties is ongoing. In all of these cases, the dispersed material is the important agent; colloidal dispersal simply facilitates its exploitation. More recently, colloidal particles have been deliberately used to modify the rheological properties of their carrier fluids. For these applications, the material from which the colloidal particles are made is less important than how their dispersal influences the medium.

This issue of MRS Bulletin focuses on a new class of applications for colloidal suspensions stemming from some particles' innate ability to organize themselves into beautifully ordered arrays. These colloidal crystals acquire interesting and useful properties not only from their constituent materials but also from the spontaneous emergence of mesoscopic order that characterizes their internal structure.

Perhaps the most remarkable observation one can make about colloidal suspensions is that they exist at all. Particles dispersed in a fluid medium have a natural tendency to aggregate under the influence of van der Waals attraction. Yet the fortunes of a great many natural and industrial processes require colloidal particles to remain dispersed or at least to aggregate at a controlled rate. The existence of colloidal suspensions as varied as milk, inks, and metallic sols attests to the efficacy of a variety of stabilizing mechanisms. As early as 1809, Russel realized that many naturally occurring colloidal particles are charged. By the end of the century, Schultz and Hardy demonstrated that the resulting electro-static repulsions were strong enough to stabilize their suspensions against flocculating. This mechanism—arguably the best understood—continues to yield new surprises despite more than a century of analysis. The most recent burst of activity has been driven by the development of new and quite general techniques for measuring colloidal and macromolecular interactions. Its counterintuitive result—that like-charged particles some-times attract each other—may have ramifications in areas as diverse as protein crystallization, self-assembly of nano-structures, and the stabilization of industrial suspensions. This article touches briefly on the well-established theory of electrostatic stabilization in colloidal suspensions. The emphasis here is on the approximations that have provided the community with an analytical theory at the expense of overlooking recently discovered effects.

Colloids have long been used in applications such as paints, coatings, foods, and many manufacturing processes. Recently, synthetic crystalline arrays of colloidal particles have been used as novel optical materials such as diffractive filters, mimicking the optical properties of opals—natural colloidal crystals made from silica spheres. Colloidal assembly has been proposed to manufacture photonic bandgap materials that can be tailored and that could have many uses in optical devices. The advantages of using colloids to do the self-assembly of novel materials are the relative ease with which monodisperse spheres comparable in size to the wavelength of light can be manufactured and also the demonstrated ease by which some suspensions of monodisperse colloidal spheres crystallize when placed under favorable conditions. Before we can use colloidal crystallization as a controlled self-assembly technique for making novel optical materials, we need (1) to create a means of manufacturing large quantities of monodisperse particles of the desired dielectric behavior, (2) to understand the phase diagram and nucleation phenomena of colloidal suspensions, and (3) to find an easy means to fix the particles in place once they selforganize. In this article, I focus on the second point just mentioned, I give an overview of the phases and some of the complex phenomena encountered in three-dimensional (3D) suspensions and in thin layers of monodisperse colloidal spheres between smooth walls, and I then briefly mention the greater complexity encountered in bidisperse systems. The first and third points will be dealt with elsewhere in this issue.

Because of their size and ability to selforganize, colloidal particles are ideal building blocks for the creation of three-dimensional (3D) structures that can have feature sizes of the order of the wavelength of electrons, photons, or both. This article is too short to provide an extensive literature survey but instead will give some illustrative examples, based on work of the author and co-workers, of how specially developed core-shell particles might be organized on a 3D lattice. These examples are only intended to give an impression of how colloidal-particle systems can be used in the design of new materials with interesting photonic properties.

Generally particles are considered colloidal if their size is between several nm and several μm. This range is more or less defined by the importance of Brownian motion—that is, the irregular, overdamped, random displacements the particles make as a result of the not completely averaged-out bombardment of solvent (or gas) molecules. Consequently the lower size range is determined by the size of the solvent molecules. Compared to the particle size, the solvent molecules need to be so small that the time scales of the solvent molecules and particles are so far apart that the solvent molecules can be “integrated out” in a description of the particles. If such a description holds, the solvent can be approximated well by a continuum. The upper size limit is determined by the size at which external fields, like gravity, start to overshadow the effects of Brownian motion.

Over the past decade, we have been working to develop intelligent photonic-crystal materials with unique properties, which will be useful in a number of technological areas. These photonic-crystal materials utilize mesoscopically periodic arrays of spherical particles as their active optical elements and are easily fabricated chemically by the use of crystalline-colloidal-array (CCA) self-assembly techniques.

Crystalline colloidal arrays are mesoscopically periodic fluid materials, which efficiently diffract light meeting the Bragg condition. These photonic-crystal materials consist of arrays of colloidal particles that self-assemble in solution into either face-centered-cubic (fcc) or body-centered-cubic (bcc) crystalline arrays (Figure 1), with lattice constants in the mesoscale size range (50-500 nm). Just as atomic crystals diffract x-rays that meet the Bragg condition, CCAs diffract ultraviolet, visible, and near-infrared light, depending on the lattice spacing; the diffraction phenomena resemble those of opals, which are close-packed arrays of monodisperse silica spheres.

The CCA however can be prepared as macroscopically ordered arrays of non-close-packed spheres. This self-assembly is the result of electrostatic repulsions between colloidal particles, each of which has numerous charged surface functional groups. We have concentrated on the development of CCAs that diffract light in the visible spectral region and generally utilize colloidal particles of ~100-nm diameter. These particles have thousands of surface charges, which result from the ionization of surface sulfonate groups. The nearest-neighbor distances are often >200 nm.