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Dielectrophoretic behaviors and assembly of a binary suspension in aqueous media are examined in the presence of nonuniform alternating current (AC) electric field. A peculiar low-frequency threshold and dielectrophoresis (DEP) crossover frequency determine the applicable frequency window for binary assembly under positive DEP, which can be effectively tuned by medium conductivity and particle size, suggesting that the dynamic double-layer effect is responsible for the interfacial polarization of micrometer to submicrometer-sized particles in aqueous suspensions. Strong effects of AC-field frequency, medium conductivity, and size ratio on binary assembly morphology have been observed. A frequency-medium conductivity phase diagram is obtained to illustrate the morphological transition of assembled colloidal aggregates from segregated, ordered assemblies to inverted segregation with the appearance of amorphous phases upon increasing frequency and/or medium conductivity, which is a direct consequence of the competition between DEP and hydrodynamic mobility. Significantly, our results demonstrate a rapid method to form hybrid nanostructured materials.

Strong frequency dependence of particle aggregation in applied ac-electric fields is reported for colloidal assembly under positive dielectrophoresis. A curious low-frequency threshold ωth on the order of 104–105 Hz is observed for the onset of collecting submicron latex particles at the high-field regions near coplanar electrodes. Similar to the dielectrophoretic crossover frequency, ωth is independent of medium conductivity, σm at low-salt concentration, yet scales with σm0.5 at high-salt concentrations. This intriguing salt effect suggests the distinct contribution from Stern and diffuse layer conductance to the interfacial polarization mechanism of latex particles in nonuniform ac fields. [ABSTRACT FROM AUTHOR]