Induced-charge electrophoresis near an insulating wall

TL;DR

This paper investigates the behavior of induced-charge electrophoresis near insulating walls under AC fields, revealing conditions for attraction, repulsion, and steady translation of particles, including Janus particles, with implications for microfluidic control.

Contribution

It extends previous ICEP analysis to AC fields and asymmetric particles, predicting new behaviors like wall attraction and steady translation, supported by experimental comparisons.

Findings

High-frequency attraction of metal cylinders to walls in AC fields.

Janus particles are always attracted to walls in AC fields.

Steady translation with tilt angle observed in experiments.

Abstract

Induced-charge electrophoresis (ICEP) has mostly been analyzed for asymmetric particles in an infinite fluid, but channel walls in real systems further break symmetry and lead to dielectrophoresis (DEP) in local field gradients. Zhao and Bau (Langmuir, \textbf{23}, 2007, pp 4053) recently predicted that a metal (ideally polarizable) cylinder is repelled from an insulating wall in a DC field. We revisit this problem with an AC field and show that attraction to the wall sets in at high frequency and leads to an equilibrium distance, where DEP balances ICEP, although, in three dimensions, a metal sphere is repelled from the wall at all frequencies. This conclusion, however, does not apply to asymmetric particles. Consistent with the recent experiments of Gangwal et al. (arXiv:0708.2417), we show that a metal/insulator Janus particle is always attracted to the wall in an AC field. The Janus particle tends to move toward its insulating end, perpendicular to the field, but ICEP torque rotates this end toward the wall. Under some conditions, the theory predicts steady translation along the wall with an equilibrium tilt angle, as seen in experiments, although more detailed modeling of the contact region of overlapping double layers is required.