Concentrated crystallization of closely packed colloidal crystal films from aqueous suspension immersed in silicone oil
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1273-MM08-06
Concentrated crystallization of closely packed colloidal crystal films from aqueous suspension immersed in silicone oil Hiroshi Fudouzi1, Tsutomu Sawada1 and Lucien Brush2 1 Optronic Materials Center, National Institute for Materials Science, Tsukuba, IB 3050047, Japan 2 Department of Materials Science & Engineering, University of Washington, Seattle, WA 98195, U.S.A. ABSTRACT Colloidal crystals have been attracting much attention due to their novel use as 3Dphotonic crystals and to their structural color. We have been developing a method for the colloidal crystal growth of opal films immersed in silicone oil. This method is one of the evaporative self-assembly techniques for opal films from colloidal particle suspensions. Understanding the mechanism of the process is important to assure the coating of high-quality opal thin films. The colloidal crystallization from suspension was observed with a long working distance optical microscope and Bragg’s diffraction peaks were measured with a miniature fiber optic spectrometer. The transition from a non-crystalline to a crystalline phase is observed within a region between the disordered colloidal suspension and the colloidal crystal film. Within this region, that spans a distance of about 400 µm, the lattice of the colloidal crystal reduces until it transitions to the close-packed structure. INTRODUCTION Colloidal crystals have attracted much attention as 3D-photonic crystals and structural color materials 1). For instance, applications that have recently been reported include 2) photonic paper/ink systems, photonic rubber sheets, solvent sensors and photonic crystal lasers. A key issue for industrial use is how to make high-quality colloidal crystal films. From a technological application viewpoint, one of the key issues is the development of a coating technique for the deposition of a high-quality colloidal crystal film onto a solid substrate. Evaporative self-assembly of colloidal particles is a typical coating technique for colloidal crystal films 3). Convective flow and capillary forces play an important role in the formation of high-quality colloidal crystal films. An alternative approach for the self-assembly process based on the phase transitions of colloidal suspension has recently been proposed and studied by the author 4). Figure 1 shows a schematic diagram of the growth process of an opal film immersed in a silicone oil layer 5). In front of the advancing colloidal crystal a non-close-packed ordered colloidal crystal first forms at the location labeled as
Figure 1. Schematic diagram of the concentrated crystallization of a colloidal crystal in silicone oil.
the interface. Then, in region ii of the schematic diagram, the distance between the colloidal particles in the non-close-packed ordered colloidal crystal continually decreases as the water continues to evaporate. Finally, the close-packed colloidal crystal opal films are formed on the substrate at the location labeled as the contact line in region iii of the diagram. This crystallization
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