Colloidal particle coordinates in three dimensions can be obtained in 3D samples with a combination of the increased resolution and optical sectioning capabilities of confocal microscopy and fluorescently labeled model core-shell silica colloids. In this work we show how this capability can be used to analyze structure formation in electrorheological fluids on a quantitative basis. We find body-centered-tetragonal (BCT) crystals for colloidal particles in an electric field. Metastable sheet like structures were identified as an intermediate phase prior to BCT crystal formation. Due to finite-size effects induced by the electrode surface the sheets are not randomly oriented, but grow preferentially with a 60degrees tilt with respect to the electric field. Preliminary measurements indicate that flow-aligned sheets form under shear. Finally, we show that in the case that the ionic strength is very low, electric-field-induced dipolar interactions can be present in addition to long-range repulsions between the colloids leading to interesting metastable and equilibrium structures with possibilities for applications in photonic bandgap crystals as well as in model ER studies.

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Journal Int. J. Mod. Phys. B
Yethiraj, A, & van Blaaderen, A. (2002). Monodisperse colloidal suspensions of silica and pmma spheres as model electrorheological fluids: A real-space study of structure formation. Int. J. Mod. Phys. B, 16, 2328–2333.