Alternating-current induced-charge electrokinetic self-propulsion of metallodielectric Janus particles in confined microchannels within a wide frequency range

Abstract

Induced-charge electrokinetics (ICEK) enables versatile microscale transport of asymmetric particles like Janus particles (JPs), but its underlying mechanisms remain incompletely understood—especially under high-frequency alternating-current (AC) fields or near confining boundaries. Prior studies often attribute JP propulsion to isolated effects: induced-charge electrophoresis (ICEP) at low frequencies or self-dielectrophoresis (sDEP) at MHz-level AC fields. They fail to integrate these mechanisms with wall effects, fluid-structure coupling, or contact mechanics, limiting predictions of real-world behaviors such as low-frequency JP attraction to insulating walls. The core novelty of this work lies in a unified 3D transient multiphysics model. It integrates previously isolated mechanisms—electrostatics, interfacial electrohydrodynamic slip, bidirectional fluid-structure interaction, and particle-wall contact mechanics—to clarify the frequency-dependent transition between ICEP and sDEP. This transition is key to characterizing dipolophoresis (DIP, frequency-tunable directed motion of polarizable particles) of both freely suspended and near-wall JPs in AC fields. Notably, the model reproduces previous benchmark experimental observations: Under low frequencies (100 Hz–10 kHz), JPs are attracted to insulating walls and exhibit 30°–60° rotational orientation toward the wall during motion. Simulations reveal three key findings: (1) In bulk fluid, JP motion reverses with frequency—dielectric-end-forward ICEP (<1 kHz) transitions to metallic-end-forward sDEP (>1 MHz)—driven by asymmetric electrochemical polarization and Maxwell–Wagner charging (the core of DIP). (2) Near insulating walls, electrohydrodynamic slip, and induced dipoles cause tilted trajectories with intermittent rotation. (3) Near electrodes, behaviors diverge by frequency: AC electroosmotic convection sustains hydrodynamic levitation at 1 kHz, while sDEP torque halts translation at 1 MHz (aligning the JP's metal–dielectric interface perpendicular to the external field). This framework resolves long-standing experimental anomalies—high-frequency JP motion cessation on electrodes and low-frequency JP attraction to insulating walls—by explicitly modeling transient surface charge redistribution, field-induced double-layer polarization, and submicron contact repulsion. These results advance predictive control of ICEK-driven particle transport, offering design principles for adaptive microfluidic systems in drug delivery and lab-on-a-chip applications.