Induced-Charge Electrophoresis of Metallodielectric Particles

The application of ac electric fields in aqueous suspensions of anisotropic particles leads to unbalanced liquid flows and nonlinear, induced-charge electrophoretic motion. We report experimental observations of the motion of Janus microparticles with one dielectric and one metal-coated hemisphere induced by uniform fields of frequency 100 Hz–10 kHz in NaCl solutions. The motion is perpendicular to the field axis and persists after particles are attracted to a glass wall. This phenomenon may find applications in microactuators, microsensors, and microfluidic devices.

Figure 1

(a) SEM image of $4.0\mu \mathrm{m}$ polystyrene particles partially coated with gold. The gold-coated hemispheres appear brighter due to their higher conductance. (b) Schematic of the experimental set-up (not drawn to scale). Two gold electrodes are deposited on the bottom plate. A Teflon spacer sustains a $60–80\mu \mathrm{m}$ gap between the bottom and the top microscope cover slip. The particle suspension is surrounded by a hydrophobic ring and confined in this thin chamber by capillarity.

Figure 2

(a) Optical micrographs from different frames of a recording of the position and orientation of Janus particles of three different diameters (4.0, 5.7 and $8.7\mu \mathrm{m}$) in an ac field of amplitude $140\mathrm{V}/\mathrm{cm}$ and frequency 1 kHz. The two particles on the right side in the top image have moved out of the field of view and another particle has moved into view in the bottom image approximately 5 s later. (b) Schematic of a particle in one-half cycle of ac electric field in the stable configuration. The electric double layer on the gold side (black hemisphere) is more strongly polarized and thus drives a stronger ICEO slip (arrows) than the polystyrene side, resulting in ICEP motion in the direction of the dielectric side.

Figure 3

Velocity of $5.7\mu \mathrm{m}$ Janus particles as a function of electric field intensity squared ($E_0{}^2$) for various NaCl concentrations at 1 kHz. The linear fits generally agree with the experimental values for low intensities and start deviating for $E_0>280\mathrm{V}/\mathrm{cm}$.

Figure 4

Average velocity of different size particles as a function of their average diameter at $300\mathrm{V}/\mathrm{cm}$ in 0.1 mM NaCl at a field frequency of 1 kHz. The data points were obtained by determining the velocity of 286 particles grouped by diameters within a $1.5\mu \mathrm{m}$ range. The line is a fit to Eq. (1) described in the main text.