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Electrokinetics at large voltages
Many microfluidic devices have been built where fluid handling is generated by electrodes that are placed close together inside the fluid channels. When electrodes are closely spaced, high electric fields can be generated at low voltage. An example is the alternating current electroosmotic (ACEO) pump, which has been analyzed and tested in the lab for several years. When an alternating array of electrodes is placed inside the channel, an ac signal applied to the electrodes can generate a steady set of vortices. Ajdari showed that if geometrical asymmetry is introduced, then the vortices are rectified to create net pumping. ACEO is part of a more general micro-fluid handling method termed induced-charge electroosmosis (ICEO). ICEO refers to systems where the double layer charge is induced on electrode surfaces by applied fields as opposed to relying on the spontaneous charge generated in normal electrokinetics. ACEO and ICEO devices are promising because they operate at low voltage which is a key feature for developing portable microfluidic systems.While many electrokinetic applications are now well-understood, ACEO and ICEO flows have been hard to analyze. In these cases, modeling has predicted the general qualitative flow structure but has failed to predict both quantitative measures and some important trends. Recent work by Bazant and co-workers has emphasized that in these application the voltages are too high for the commonly used Poisson-Boltzman (PB) theory to be valid. One breakdown in the theory is that at high voltages the finite sizes of ions becomes important. This deficiency of PB has been known for about 100 years and many, many papers have been have written on a variety of corrections. However, microfluidics presents a new arena for such theories. Microfluidic applications can involve complex fluid flow and temporal dynamics, which are not present in most previous studies where only the equilibrium problem is considered.
We started to explore how these new double layer models that account for steric effects can impact the calculated flows in ACEO/ICEO devices. Some of our results (see PRE 2008) have shown a dramatic change in the flow behavior when one considers steric effects, including a prediction of flow reversal which has been experimentally observed but never predicted by any model. As we started to learn more about past attempts to rectify such problems in the classic theory, we learned more about a diverse and extensive literature. This work led us to write an extensive review article which covers a number of additional problems in the classical PB theory. This article covers the history of attempts to go beyond PB and some of the possible ramifications for microfluidic applications.
More recently, we have been exploring one of the most basic assumptions, the mean-field approximation. The mean-field approximation says that the electric field acting on an individual ion is self-consistently determined by the local mean charge density. In many situations, such as concentrated electrolytes, multivalent electrolytes, or solvent-free ionic liquids, the mean-field approximation breaks down. We have developed and tested a fourth-order modified Poisson equation that captures the essential features in a simple continuum framework. We can reproduce data from room temperature ionic liquids quite well.
funding: National Science Foundation (abstract)
Student researchers: Jacqueline Baca, Lee Edwards.
Collaborators: Martin Bazant at MIT,
Sabri Kilic (graduate student at MIT), Armand Ajdari.
Publications:
- Storey, B.D., and Bazant, M.Z. 2012 Effect of diffuse layer electrostatic correlations on electrokinetic phenomena. Physical Review E 86, 056303. (Full Text) (arXiv)
- Bazant, M.Z., Storey, B.D., and Kornyshev A. A., 2011 Double layer in ionic liquid: overscreening vs. crowding. Physical Review Letters 106, 046102. (Full Text, ArXiv).
- Bazant, M.Z., Kilic, M.S., Storey, B.D. & Ajdari, A., 2009 Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions. Advances in Colloid and Interface Science, 152, 48-88. ( Full Text)
- Bazant, M.Z., Kilic, M.S., Storey, B.D. & Ajdari, A. 2009 Nonlinear electrokinetics at large voltages. New Journal of Physics, 11, 075016. ( Full Text)
- Storey, B.D., Edwards, L.R., Kilic, M.S., & Bazant, M.Z. 2008 High-frequency flow reversal of AC electro-osmosis due to steric effects. Physical Review E 77, 036317. (Full Text)
- Bazant, M.Z., Kilic, M.S., Storey, B.D. & Ajdari, A. 2007 Nonlinear electrokinetics at large applied voltages. (arXiv).