Danish Technical University, October 23, 2006
Nonlinear electrokinetics at large voltages
Martin Z. Bazant (Department of Mathematics, MIT)
Abstract:
Beginning with the recent discovery of AC electro-osmosis at
micro-electrode arrays, much attention has been paid to nonlinear
"induced-charge" electrokinetic phenomena. Other examples include DC
or AC flows around polarizable (metal or dielectric) microstructures
and induced-charge electrophoretic motion of polarizable colloids. The
current theory is borrowed from classical linear electrokinetics in a
dilute solution, which can only be justified for "small" surface
potentials (of order kT/e = 25 mV). In all situations of interest,
however, much larger voltages are applied across the double layer, up
to several volts = 100 kT/e, so that the solution cannot remain
dilute. As a result, the theory fails to predict crucial experimental
features, such as the loss of flow at high ionic strength (> 10 mM)
and flow reversal at high voltage and high frequency (in certain
geometries). This talk presents a modified theory taking into account
(i) steric effects for ions of finite size and (ii) nonlinearities in
viscosity and permittivity, which suffices to explain some (but not
all) of the experimental puzzles.