AC Electrokinetic micropumps

The goal of the present thesis has been to investigate ways of controlling
and manipulating fluids and suspensions in microfluidic systems. In particular, we focus on a theoretical description of AC electroosmotic micropumps
with asymmetric electrode arrays, that have recently been demonstrated to
permit fast pumping (velocities ∼ mm/s) with low driving voltage of a few
volt only.
The dynamical description of electrokinetics and electrochemical transport at driving voltages of just a few volt is a theoretically challenging
subject, and therefore simplifying assumptions such as the Debye–H¨uckel
approximation or linear response for weak applied field have often been employed in the literature.
We extend previous linear theory for AC electroosmotic flow into the
“weakly nonlinear” regime by accounting for nonlinear capacitance of the
Debye screening layer, and also consider the effect of Faradaic current injection from electrochemical electrode reactions. This allows us to explain
why the frequency of maximum pumping is sometimes shifted down when
the driving voltage is increased, but neither the linear nor weakly nonlinear
models are able to account for the reversal of the pumping direction that
has been observed experimentally.
Therefore we also study the “strongly nonlinear” regime where classical
circuit models with uniform bulk electrolyte concentration break down. We
extend recent theoretical work in this regime, by accounting for dynamics
in the diffusion layer developing when an AC voltage with driving frequency
around the inverse RC time is applied, and by considering fluid motion and
convection of ions. Moreover, we attempt to include existing theory for
double layers driven out of quasiequilibrium from problems of DC Faradaic
conduction at “very large” (but experimentally relevant) voltage into our
dynamical model.
We solve the coupled electrohydrodynamical problem numerically for
experimental micropump geometries and display contributions to the net
pumping velocity from the different flow sources in the model: Our results
indicate that both bulk electroconvection and electroosmotic flow of the
“second kind” may contribute a significant fraction of the overall induced
flow already at driving voltages of a few volt. However, a rigorous account for nonequilibrium double layers in a dynamical setting would be necessary
to justify several ad hoc assumptions in our model.
Finally, we investigate different ways of breaking the symmetry of an
electrode array and determine in each case the optimal device geometry to
maximize the pumping velocity at a given low driving voltage as described
by the simple linear theory