Bifurcation in the Steady-State Height of Colloidal Particles near an Electrode in Oscillatory Electric Fields: Evidence for a Tertiary Potential Minimum

Abstract

Application of an oscillatory electric field is known to alter the 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 flow generated around each particle, with previous work focused on identifying a single steady-state "height" of the individual particles over the electrode. Here, we report the existence of a pronounced bifurcation in the particle height in response to low-frequency electric fields. Optical and confocal microscopy observations reveal that application of a ∼100 Hz field induces some of the particles to rapidly move several particle diameters up from the electrode, while the others move closer to 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, in a fashion qualitatively consistent with an energy landscape predicated on competition between electrohydrodynamic flow, colloidal interactions, and gravitational forces. Taken together, the results provide evidence for the existence of a deep tertiary minimum in the effective electrode-particle interaction potential at a surprisingly large distance from the electrode.

Polymer and Particle Dynamics and Assembly in Varied Hydrodynamic Fields

“…This separation is classified by Figure b as “Up” and “Down” particles, with the addition of a third “Stuck” class that occurs when particles irreversibly attach to the electrode. This bifurcation has been experimentally observed, shown in Figure d with false‐colored confocal imagery of the two populations and resolved particle heights . A possible explanation for this bifurcation is given in Figure c, which predicts secondary and tertiary potential minimums above the surface of the electrode for NaOH, based on contributions from colloidal‐scale forces and EHD forces.…”

“…Complicating the understanding of particle behavior near an electrode are two additional observations, including an electrolyte effect, and a bimodal particle height distribution . Figure a quantifies the rate of aggregate formation, k E, for various electrolytes .…”

“…c) Particle–electrode interaction potentials for a 2 μm sulfated PS particle in 1 × 10 −3 m NaOH with a 4 Vpp, 100 Hz electric field applied and d) representative confocal microscopy images of the height bifurcation to two distinct focal planes after the application of the electric field. Reproduced with permission . Copyright 2015, American Physical Society.…”

Thermodynamics and dynamics of two-dimensional systems with dipolelike repulsive interactions

“…An external electric field polarizes the particles and ion clouds in the solvent around them, inducing a (tunable) dipole-dipole interaction between the particles. Depending on the orientation of the external electric field with respect to the plane of particle confinement, the dipolar interaction potential can be either attractive [24][25][26][27] or repulsive [28][29][30][31][32][33][34].…”

Electrohydrodynamic Deformation and Rotation of a Particle-Coated Drop

“…Electric fields provide a versatile means to control small-scale fluid and particle dynamics, e.g., electrohydrodynamic instabilities for pattern formation in thin polymer films [1][2][3] or particle suspensions [4], electrohydrodynamic atomization to produce micro-and nanoparticles [5][6][7][8][9], drop and vesicle manipulation [10,11], and colloidal assembly [12][13][14][15][16][17][18].…”