Extreme Levitation and Height Bifurcation of Colloidal Particles in Response to Oscillatory Electric Fields*

Micron-scale colloidal particles in solution exhibit a variety of behaviors in response to oscillatory electric fields, including aggregation, separation, and a distinct bifurcation in the average particle height above the electrode. For unclear reasons, all of these behaviors are strongly dependent on the identity of the surrounding electrolyte. Here we use confocal microscopy to demonstrate that at sufficiently low frequencies particles suspended in certain electrolytes exhibit extreme levitation away from the electrode surface, reaching heights greater than 30 particle diameters.  The levitation height is shown to scale as the inverse square root of the frequency for both NaOH and KOH solutions, with weak dependence on the magnitude of the applied field and the particle size.  We demonstrate that the particle behavior is consistent with a mechanism based on asymmetric rectified electric fields (AREFs) recently proposed by Hashemi Amrei et al., and we discuss the implications for manipulating particles using controlled levitation.