Induced-charge electrophoresis near a wall.

Induced-charge electrophoresis (ICEP) has mostly been analyzed for asymmetric particles in an infinite fluid, but channel walls in real systems further break symmetry andlead to dielectrophoresis (DEP) in local field gradients. Zhao and Bau (Langmuir 2007, 23,4053) 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 experiments of Gangwal et al. (Phys. Rev. Lett. 2008, 100, 058302), 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, perpendicular to the field, at an equilibrium tilt angle around 451, consistent with the experiments, although improved models are needed for a complete understanding of this phenomenon.