Three Dimensional Confocal Microscopy Study of Boundaries between Colloidal Crystals Harvard University, School of Engineering and Applied Science, Cambridge, MA 02138 ABSTRACT Colloidal crystals were grown on flat or patterned glass slides. The structure of the grains and their defects was first visualized by 3D confocal microscopy and then characterized using simple geometric measurements. Crystals grown on a flat surface maintained a layered structure induced by the closed-packed planes. In the case of the [110] 5 grain boundary, the presence of particles in interlayer position was established. 1 Introduction The grain-level microstructure of a material influences a wide range of material properties, including strength, toughness and corrosion resistance. For that reason, understanding and controlling the structure and evolution of grain boundaries is one of the central tasks of materials science. Studying grains at the atomic level moreover, is not an easy task. To aid in this, we used colloidal suspensions as model systems that form crystals. In recent research, colloids have been used to model atomic or molecular systems since they form many of the same phases. They can be used for model glasses as well as crystals [1, 2]. A colloidal system has two distinct phases: a dispersed phase and a continuous one. The dispersed phase consists of small solid particles, on a nano to macro scale, which are dispersed evenly through the continuous fluid phase. The particles used in this study interact as hard spheres. When these particles sediment onto a flat surface, they can form crystals; when they sediment onto an irregular, rough surface, they can form amorphous structures. For the random close-packing, the structural paradigm for the amorphous phase, the volume fraction of solid, f, is about 0.63, and for the close-packed crystals, f is about 0.74. The crystals often contain defects and the focus of the present paper is on grain boundaries. The main purpose of the paper is to show how the crystals can be grown and imaged in three dimensions (3D) using confocal microscopy. 2 Experimental procedure 2.1 Colloidal suspension Silica particles (diameter 1.55 μm, density 2.0 g/cm3, mass 3.9 x 10-15 kg) were suspended in a water - 62.8 vol. % dimethylsulfoxide (DMSO) solution that matched the index of refraction of the silica and had a density of 1.10 g/cm3. The average velocity of Brownian motion is given by: <vB>= (3kB/Tm) 1/2 (1) where kB is Boltzmann's constant, T the temperature and m the mass of the particle. For our case, this gives <vB>=2 x 10 -3 m/s. The gravitational settling velocity of the particle is given by vS = VP Δρ g / 6π r η (2) where VP and r are, respectively, the volume and radius of the particle, Δρ is the density difference between particle and fluid, and η is the viscosity of the fluid, which is about 10-2 Pa.s. This gives in our case v S=10-6 m/s, which satisfies the condition for a colloidal systems (vB>>vs). The index match makes the system optically transparent, which allows investigation by optical confocal microscopy at large distances into the sample. Contrast between particle and solution was achieved by adding fluorescein dye to the solution. The index match also minimizes the van der Waals forces between the particles, which interact therefore like hard spheres. 1 Present address : INSA-Lyon, MATEIS UMR5510, 25 av. Capelle, 69621 Villeurbanne, France 2 Present address : Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 5608531, Japan T. Proulx (ed.), Optical Measurements, Modeling, and Metrology, Volume 5, Conference Proceedings of the Society for Experimental Mechanics Series 9999999, DOI 10.1007/978-1-4614-0228-2_10, © The Society for Experimental Mechanics, Inc. 2011 69 Σ E. Maire1, M. Persson Gulda, N. Nakamura2, K. Jensen, E. Margolis, C. Friedsam F. Spaepen
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