Double-Layer AC Tangential Conduction Effects on Polarization, Assembly and Dielectrophoretic Mobility of Particles in Ionic Liquids*

Polarization of gas bubbles and liqud drops by an AC field in ionic liquids is shown to exhibit anomalous induced-dipole behaviors that cannot be explained by classical Maxwell-Wagner dielectric polarization theories. A dielectrophoretic cross-over frequency exists even though the ionic liquid has much higher permittivity and conductivity than the particle, even when conducting Stern layer effects are included. Moreover, the cross-over frequency is observed to be field and particle size dependent. Drops of different size can hence have anti-parallel induced dipoles and are observed to coalesce at an intermediate frequency between the cross-over frequencies of the two particles. ~A scaling theory based on a thin-layer tangential conduction model is found to collapse our measured cross-over frequency and dielectrophoretic velocity data.