(214c) A Deep Tertiary Minimum in the Particle/Electrode Interaction Energy in Oscillatory Fields

Application of an oscillatory electric field is known to alter the equilibrium separation distance between micron-scale colloidal particles and an adjacent electrode.  This behavior is believed to be partially due to a lift force caused by electrohydrodynamic (EHD) flow generated around each particle, with previous work focused on identifying a single equilibrium “height” of the individual particles over the electrode.  Here we report the existence of a pronounced bifurcation in the equilibrium particle height in response to low frequency electric fields.  Optical and confocal microscopy observations reveal that application of a 50 - 300 Hz field induces some of the particles to rapidly move several particle diameters up from the electrode, while the others remain near the electrode.  Statistics compiled from repeated trials demonstrate that the likelihood for a particle to move up follows a binomial distribution, indicating that the height bifurcation is random and does not result from membership in some distinct subpopulation of particles. The fraction of particles that move up increases with increased applied potential and decreased frequency.  Taken together, the results provide evidence for the existence of a deep tertiary minimum in the electrode-particle interaction energy at a surprisingly large distance from the electrode.