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Structure, Stability and Applications of Colloidal Crystals Korea-Australia Rheology Journal Vol. 20, No. 3, September 2008 pp. 97-107 Structure, stability and applications of colloidal crystals Masaki Yanagioka and Curtis W. Frank* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 (Received April 18, 2008) Abstract This article presents an overview of current research activities that center on colloidal crystals resulting from self-assembly of surface-charged nanoparticles. It is organized into three parts: the first part discusses characterization of colloidal structures, the second part describes colloidal stability from the rheological aspects of colloidal crystals suspended in medium, and the third part highlights polymerized colloidal crystals as a promising application. Finally, we briefly discuss the directions of future research in this area. Keywords : colloidal crystals, stability of colloids, crystal structure, rheology 1. Introduction ticle becomes unscreened and is enhanced, resulting in electrostatic repulsion among the particles. Because the Structure and physical properties of colloids have been volume of the colloidal dispersion is limited, the particles studied intensively, since colloidal systems are important in take the lowest possible energy configuration, which is a various fields of industry and as models of solid and liquid highly ordered crystal structure. Since this electrostatic structures for the study of phase transition behavior repulsion exerts a long-range force upon the particles, the (Medeiros e Silva and Mokross, 1980; Hao, 2005). The average interparticle distance can be as long as the order of most studied colloidal systems consist of monodisperse the wavelength of light, depending on the volume fraction nanoparticles, which spontaneously self-assemble under and the electrolyte concentration in the medium. Therefore, appropriate conditions. Systems of these monodisperse these colloidal crystals exhibit iridescence arising from the particles exhibit interesting diffraction phenomena and are Bragg diffraction of visible light, and light scattering can referred to as colloidal crystals. The colloidal crystals are be used to characterize the structural ordering. Compared roughly divided into close-packed and non-close-packed to the close-packed crystals, the non-close-packed colloids categories. The close-packed crystals comprise particles are formed at much smaller volume fraction (Pusey and that are in contact with each other, and one of the van Megen, 1986). approaches to form a close-packed crystal is to utilize In this review, we first discuss characterization of col- evaporation-induced self-assembly driven by capillary loidal structures because they form a basis for all properties forces (Gu et al., 2002; Nakamura et al., 2006). The shim- of colloidal crystals. Next we describe the stability of the mering wings of butterflies and iridescent opals are exam- colloidal crystals from the viewpoint of their rheological ples of the close-packed crystals found in nature. properties. Then we will highlight polymerized colloidal Non-close-packed crystals are achieved by enhancing the arrays, where colloidal crystals are embedded in a poly- surface charge on the particles such that they electrostat- meric matrix. Finally, we conclude with some comments ically repel one another. Here, colloidal crystals are on the directions towards which future research in this area regarded as being soft due to the surrounding electrical might be focused. double layer, which plays an important role in the ordering process. Aqueous suspensions of colloidal polystyrene or 2. Discussions silica particles are good examples of such suspensions. Typically, these particles are negatively charged, with 2.1. Colloidal structure counterions from the medium adsorbed onto the particles Any detailed understanding of interparticle interactions to neutralize the surface charge. By removing the coun- in colloidal suspensions will require the determination of terions in the bulk solution using dialysis and ion-exchange the crystal structure. Depending on the volume fraction and resin (Goodwin et al., 1980), the surface charge on the par- the salt concentration, the suspension can undergo order- disorder transitions from a fluid state to a highly organized *Corresponding author: [email protected] crystal of either body-centered cubic (BCC) or face- © 2008 by The Korean Society of Rheology centered cubic (FCC) structure. Since the characterization Korea-Australia Rheology Journal September 2008 Vol. 20, No. 3 97 Masaki Yanagioka and Curtis W. Frank of colloidal structures was first discussed in detail using Bragg diffraction by Hiltner and Krieger (1969), intensive research has been conducted on colloidal crystals. Deter- mination of the colloidal structure has been achieved by Bragg diffraction (Reese et al., 2001), Kossel diffraction (Kremer et al., 1986; Robbins et al., 1988; Rundquist et al., 1989; Monovoukas and Gast, 1989), light scattering (Heimer and Tezak, 2002), or x-ray scattering (Matsuoka et al., 1991; Konishi and Ise, 2006; Men et al., 2006; Reichelt et al., 2008), and some of the representative studies are reviewed in this paper. 2.1.1. Kossel diffraction Generally, irradiation of the ordered colloidal crystal dispersion with visible light leads to Bragg diffraction. In addition to Bragg diffraction, Carlson and Asher (1984) took advantage of the so-called Kossel rings in order to determine the colloidal structure. The laser light scatters isotropically within the sample, but some portion of the scattered light that satisfies the Bragg angle cannot propagate through the sample. The dark Kossel rings, thus, Fig. 2. Experimental apparatus showing Kossel ring patterns. result from this inability of the scattered light to propagate Reprinted with permission from (Carlson and Asher, along directions that satisfy the Bragg condition. In 1984). Copyright © 1984 Society for Applied Spec- addition to the dark Kossel rings, multiple Bragg troscopy. diffraction leads to formation of the broadened bright Kossel rings, which are located outside the dark Kossel suspension of polystyrene spheres using dialysis and ion- rings. The mechanism of the Kossel ring formation is exchange resin. Carlson and Asher (1984) placed a shown in Fig. 1. capillary that contained the colloidal crystal dispersion in a Colloidal crystal dispersion of polystyrene spheres was cylindrical sample container and irradiated the capillary formed by removing excess ions from an aqueous with argon-ion laser (Fig. 2). On the wall of the container, they observed a series of Kossel rings that accompanied the Bragg diffraction spots. These Kossel rings are the trajectories of dark lines resulting from the intersection of surfaces of cones with the cylindrical screen around the sample chamber. The light is scattered within the sample; therefore, the apex of each cone also locates in the sample. Investigation of these rings enabled Carlson and Asher to determine simultaneously both the crystal structure and orientation. They demonstrated that the interparticle dis- tance in the crystals was many times larger than the sphere diameter and that the electrostatic repulsion was indeed the driving force of the formation of the non-close-packed col- loidal crystals. 2.1.2. Fluorescence confocal scanning laser micro- scope (CSLM) In the 1990’s, a new type of light microscope, a fluorescence confocal scanning laser microscope (CSLM), Fig. 1. Formation of bright and dark Kossel rings. Isotropic scat- was introduced into colloid science to study concentrated tering of the laser light results in a divergent light source within the sample. The light at the Bragg angle θ is dif- colloidal crystals. Verheagh et al. (1995) characterized fracted and cannot propagate through the lattice. Thus, colloidal systems of sterically stabilized silica spheres that dark Kossel rings are projected onto a target screen. The crystallized into close-packed structures. Structure analysis bright Kossel rings are considerably broadened by mul- by conventional light microscopy is not useful for these tiple Bragg diffraction. Reprinted with permission from concentrated colloidal crystals, since the intensity of light (Carlson and Asher, 1984). Copyright © 1984 Society for is strongly reduced by scattering. The CSLM rejects light Applied Spectroscopy. that does not come from the focal plane, and this permits 98 Korea-Australia Rheology Journal Structure, stability and applications of colloidal crystals Fig. 4. CLSM image after averaging over 20 frames shows coex- istence of voids (dark region) with dense disordered regions (gray region) for the highly charged polystyrene particles taken at a distance of 60 µm from the cover glass. Images are taken using λ= 488 nm of Ar-ion laser and a 40 × /0.75 objective. Scale bar = 20 µm. Reprinted with permission from (Mohanty et al., 2005). Copyright © 2005 American Chemical Society. Fig. 3. Confocal scanning laser micrographs of a polycrystalline spherical colloidal particles. The static structure factor S(Q) optical section in a 7 vol% dispersion of rhodamine- enables us to identify the colloidal structure. labeled silica spheres in chloroform. (a) Micrograph taken Compared to the light scattered by the colloidal particles, at a depth of about 20 µm below the glass wall. (b) Micro- the light scattered by the solvent is negligibly smaller. The graph taken 0.4 µm below the previous one. Scale bar = 10 µm. Reprinted with permission from (Verhaegh et al., following Rayleigh-Gans
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