  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.
