THE formation of patterned colloidal structures from dispersions of particles has many potential uses in materials processing1–3Structures such as chains of particles that form in the presence of electric or magnetic fields are also central to the behaviour of electrorheological fluids4–6 and ferrofluids7Electrohydrodynamic effects in aqueous suspensions have been described by Rhodes et al.8Here we show that such effects can be used to create structures within a non-aqueous colloidal dispersion of dielectric particles. When the conductivity of a particle-rich spherical region (bolus) is higher than that of the surrounding fluid, an electric field deforms the bolus into a prolate ellipsoid. If the conductivities are reversed (by adding salt to the surrounding fluid, for example), a disk-like shape results. In this way, we form colloidal columns, disks and more complex structures. Once formed, these could be frozen in place by solidifying the fluid matrix by gelation or polymerization9.
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