Externally directed assembly of disk-shaped zeolite particles by an electric field
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Alternating current (AC) electric fields generated by coplanar electrodes are used to externally direct the assembly of submicrometer sized disk-shaped zeolite particles. At the edge of the electrode, zeolite particles assemble in a brushlike structure that forms because of an interplay between an induced dipolar interaction and the drag force due to AC electroosmotic flow. Far from the electrode edge, where the fluid is quiescent, the disk-shaped particles form a nearly hexagonally close-packed structure, similar to suspensions of spherical particles. These results demonstrate a surprising generality of field-directed structures and offer promise for a hierarchical fabrication of nanostructures from zeolites.
I. INTRODUCTION
Externally directed assembly of colloidal suspensions promises to be a key paradigm in the bottom-up fabrication of functional micro- and nanostructures.1 Electric fields generate induced dipolar interactions between particles, which gives rise to a tunable and reversible disorder-toorder transition in the case of model suspensions of spheres.2,3 Depending on the electrode geometry and field strength, the induced conservative interactions can further couple to local electrohydrodynamic (EH) flows, yielding a rich array of organized microstructural states.4–7 It is only recently, however, that the directed assembly of other particle shapes and sizes has begun to be explored. Anisotropic shapes, such as ellipsoids, lead to the self-assembly of different symmetries and more open structures,8 and imposing externally directed flow fields in addition to polarization forces has recently been used to tailor the optical and mechanical properties of colloidal structures.9–11 Nonetheless, a general understanding of the influence of particle shape on the assembly of colloidal particles remains to be developed. With this in mind, in this paper we present the directed assembly of disk-shaped zeolite particles in alternating current (AC) electric fields. Zeolites are crystalline materials with uniform, nanopore networks. This unique pore structure gives these materials the ability to act as molecular sieves, and the high acidity of zeolite particles also makes them catalytically active.12 In addition to naturally occurring zeolites, a large number of zeolites have been fabricated with sizes ranging from nanometer to micrometer dimensions and with both isotropic and anisotropic shapes.13,14 In most applications zeolites are employed as films, pellets, or powders. In the resulting
macrostructure the pores are oriented randomly and a continuous pore network is not preserved. It was recently shown, however, that zeolite particles packed in an ordered structure in which the pores are oriented in one direction have a higher selectivity in the separation of xylene isomers.15 Furthermore, because of a wide range of shape and sizes, colloidal zeolites also serve as precursors for the self-assembly of functional materials.16,17 Oriented zeolite crystals have been obtained by growing zeolite particles in ordered arrays on a chemi
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