Double-layer effects on low frequency dielectrophoresis-induced colloidal assembly

Abstract

Strong frequency dependence of particle aggregation in applied ac-electric fields is reported for colloidal assembly under positive dielectrophoresis. A curious low-frequency threshold ${\omega }_{\mathrm{th}}$ on the order of ${10}^4–{10}^5\mathrm{Hz}$ is observed for the onset of collecting submicron latex particles at the high-field regions near coplanar electrodes. Similar to the dielectrophoretic crossover frequency, ${\omega }_{\mathrm{th}}$ is independent of medium conductivity, ${\sigma }_m$ at low-salt concentration, yet scales with ${\sigma }_m^{0.5}$ 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.