Hypothesis
The interaction of active particles with walls can explain discrepancies between experiments and theory derived for particles in the bulk. For an electric field driven metallodielectric Janus particle (JP) adjacent to an electrode, interaction between the asymmetric particle and the partially screened electrode yields a net electrostatic force – termed self-dielectrophoresis (sDEP) - that competes with induced-charge electrophoresis (ICEP) to reverse particle direction.
Experiments
The potential contribution of hydrodynamic flow to the reversal is evaluated by visualizing flow around a translating particle via micro-particle image velocimetry and chemically suppressing ICEP with poly(l-lysine)-g-poly(ethylene glycol) (PLL-PEG). Mobility of Polystyrene-Gold JPs is measured in KCl electrolytes of varying concentration and with a capacitive SiO2 coating at the metallic JP surface or electrode. Results are compared with theory and numerical simulations accounting for electrode screening.
Findings
PLL-PEG predominantly suppresses low-frequency mobility where propulsive electro-hydrodynamic jetting is observed; supporting the hypothesis of an electrostatic driving force at high frequencies. Simulations and theory show the magnitude, direction and frequency dispersion of JP mobility are obtained by superposition of ICEP and sDEP using the JP height and capacitance as fitting parameters. Wall proximity enhances ICEP and sDEP and manifests a secondary ICEP charge relaxation time dominating in the contact limit.
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This content will become publicly available on December 1, 2025
Active microparticle propulsion pervasively powered by asymmetric AC field electrophoresis
Hypothesis: Symmetry breaking in an electric field-driven active particle system can be induced by applying a
spatially uniform, but temporally non-uniform, alternating current (AC) signal. Regardless of the type of particles
exposed to sawtooth AC signals, the unevenly induced polarization of the ionic charge layer leads to a major
electrohydrodynamic effect of active propulsion, termed Asymmetric Field Electrophoresis (AFEP).
Experiments: Suspensions containing latex microspheres of three sizes, as well as Janus and metal-coated particles
were subjected to sawtooth AC signals of varying voltages, frequencies, and time asymmetries. Particle tracking
via microscopy was used to analyze their motility as a function of the key parameters.
Findings: The particles exhibit field-colinear active propulsion, and the temporal reversal of the AC signal results
in a reversal of their direction of motion. The experimental velocity data as a function of field strength, frequency,
and signal asymmetry are supported by models of asymmetric ionic concentration-polarization. The
direction of particle migration exhibits a size-dependent crossover in the low frequency domain. This enables
new approaches for simple and efficient on-chip sorting. Combining AFEP with other AC motility mechanisms,
such as induced-charge electrophoresis, allows multiaxial control of particle motion and could enable development
of novel AC field-driven active microsystems.
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- Award ID(s):
- 2133983
- PAR ID:
- 10540413
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Colloid and Interface Science
- Volume:
- 676
- Issue:
- C
- ISSN:
- 0021-9797
- Page Range / eLocation ID:
- 817 to 825
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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