Seminar: Peter Ehrhard, Dortmund Technical University, Germany
Professor Peter Ehrhard of Dortmund Technical University, Germany presents a seminar in Chemical & Biomolecular Engineering, "Simulations and experiments on electrokinetic flows in microchannels with internal electrodes."
Abstract: In microfluidic devices, electrokinetic effects - in general - can be engaged to set up flows which may pump or mix the fluids. We focus on the effect of internal electrodes onto the flow field in a modular rectangular microchannel. As internal electrodes can be positioned at low distances, they can be operated at low voltages and still ensure strong electrical fields. However, electrode reactions may influence the species concentration, and reaction products may be kept in dissolution or may be released in gaseous form.
The mathematical model for the theoretical treatment relies on a first-principle description of the EDL and the electrical forces caused by the electrical field between the internal electrodes. Hence, the so-called Debye-Hückel approximation is avoided. The governing system of equations consists of a Poisson equation for the electrical potential, the continuity and Navier-Stokes equations for the flow field, species transport equations, based on the Nernst-Planck equation, and a charge transport equation. Further, a model for the electrode reactions, based on the Butler-Volmer equation, is in place. The simulations are time-dependent and two-dimensional (plane) in nature and employ a finite-volume method (FVM).
The validation experiments engage rectangular microchannels of 100 x 200 mm cross section. In each microchannel eight pairs of electrodes are placed onto the top and bottom glass covers, whereas the offset of the electrodes is chosen differently in each of the microchannels. Due to the limited optical access through the glass covers, only two velocity components are accessible by means of the micro-particle-image velocimetry (mPIV). Multiple measuring planes and an integration of the continuity equation allow for the reconstruction of the third velocity component at reasonable accuracy. Further, the microparticles used for mPIV are electrically non-neutral. Hence, in addition to drag forces from fluid flow, they experience electrophoretic forces. Hence, the challenge arises to infer the fluid flow from the particle movement.