AC Electrokinetic micropumps

Laurits Højgaard Olesen

    Research output: Book/ReportPh.D. thesis

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    Abstract

    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
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages184
    ISBN (Print)87-89935-81-0
    Publication statusPublished - 2006

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