Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions
The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of point-like ions in quasi-equilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e ≈ 25 mV at room temperature). In nonlinear electrokinetic ph...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | en_US |
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Elsevier
2012
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| Online Access: | http://hdl.handle.net/1721.1/69106 |
| _version_ | 1826211226278952960 |
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| author | Bazant, Martin Z. Kilic, Mustafa Sabri Storey, Brian D. Ajdari, Armand |
| author2 | Massachusetts Institute of Technology. Department of Chemical Engineering |
| author_facet | Massachusetts Institute of Technology. Department of Chemical Engineering Bazant, Martin Z. Kilic, Mustafa Sabri Storey, Brian D. Ajdari, Armand |
| author_sort | Bazant, Martin Z. |
| collection | MIT |
| description | The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of point-like ions in quasi-equilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e ≈ 25 mV at room temperature). In nonlinear electrokinetic phenomena, such as AC or induced-charge electro-osmosis (ACEO, ICEO) and induced-charge electrophoresis (ICEP), several V ≈ 100 kT/e are applied to polarizable surfaces in microscopic geometries, and the resulting electric fields and induced surface charges are large enough to violate the assumptions of the classical theory. In this article, we review the experimental and theoretical literatures, highlight discrepancies between theory and experiment, introduce possible modifications of the theory, and analyze their consequences. We argue that, in response to a large applied voltage, the “compact layer” and “shear plane” effectively advance into the liquid, due to the crowding of counterions. Using simple continuum models, we predict two general trends at large voltages: (i) ionic crowding against a blocking surface expands the diffuse double layer and thus decreases its differential capacitance, and (ii) a charge-induced viscosity increase near the surface reduces the electro-osmotic mobility; each trend is enhanced by dielectric saturation. The first effect is able to predict high-frequency flow reversal in ACEO pumps, while the second may explain the decay of ICEO flow with increasing salt concentration. Through several colloidal examples, such as ICEP of an uncharged metal sphere in an asymmetric electrolyte, we show that nonlinear electrokinetic phenomena are generally ion-specific. Similar theoretical issues arise in nanofluidics (due to confinement) and ionic liquids (due to the lack of solvent), so the paper concludes with a general framework of modified electrokinetic equations for finite-sized ions. |
| first_indexed | 2024-09-23T15:02:53Z |
| format | Article |
| id | mit-1721.1/69106 |
| institution | Massachusetts Institute of Technology |
| language | en_US |
| last_indexed | 2024-09-23T15:02:53Z |
| publishDate | 2012 |
| publisher | Elsevier |
| record_format | dspace |
| spelling | mit-1721.1/691062022-09-29T12:16:20Z Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions Bazant, Martin Z. Kilic, Mustafa Sabri Storey, Brian D. Ajdari, Armand Massachusetts Institute of Technology. Department of Chemical Engineering Massachusetts Institute of Technology. Department of Mathematics Bazant, Martin Z. Bazant, Martin Z. Kilic, Mustafa Sabri The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of point-like ions in quasi-equilibrium with a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage (kT/e ≈ 25 mV at room temperature). In nonlinear electrokinetic phenomena, such as AC or induced-charge electro-osmosis (ACEO, ICEO) and induced-charge electrophoresis (ICEP), several V ≈ 100 kT/e are applied to polarizable surfaces in microscopic geometries, and the resulting electric fields and induced surface charges are large enough to violate the assumptions of the classical theory. In this article, we review the experimental and theoretical literatures, highlight discrepancies between theory and experiment, introduce possible modifications of the theory, and analyze their consequences. We argue that, in response to a large applied voltage, the “compact layer” and “shear plane” effectively advance into the liquid, due to the crowding of counterions. Using simple continuum models, we predict two general trends at large voltages: (i) ionic crowding against a blocking surface expands the diffuse double layer and thus decreases its differential capacitance, and (ii) a charge-induced viscosity increase near the surface reduces the electro-osmotic mobility; each trend is enhanced by dielectric saturation. The first effect is able to predict high-frequency flow reversal in ACEO pumps, while the second may explain the decay of ICEO flow with increasing salt concentration. Through several colloidal examples, such as ICEP of an uncharged metal sphere in an asymmetric electrolyte, we show that nonlinear electrokinetic phenomena are generally ion-specific. Similar theoretical issues arise in nanofluidics (due to confinement) and ionic liquids (due to the lack of solvent), so the paper concludes with a general framework of modified electrokinetic equations for finite-sized ions. National Science Foundation (U.S.) (contract DMS-0707641) 2012-02-15T14:06:01Z 2012-02-15T14:06:01Z 2009-10 Article http://purl.org/eprint/type/JournalArticle 0001-8686 http://hdl.handle.net/1721.1/69106 Bazant, Martin Z. et al. “Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions.” Advances in Colloid and Interface Science 152.1-2 (2009): 48-88. en_US http://dx.doi.org/10.1016/j.cis.2009.10.001 Advances in Colloid and Interface Science Creative Commons Attribution-Noncommercial-Share Alike 3.0 http://creativecommons.org/licenses/by-nc-sa/3.0/ application/pdf Elsevier Prof. Bazant via Erja Kajosalo |
| spellingShingle | Bazant, Martin Z. Kilic, Mustafa Sabri Storey, Brian D. Ajdari, Armand Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title | Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title_full | Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title_fullStr | Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title_full_unstemmed | Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title_short | Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions |
| title_sort | towards an understanding of induced charge electrokinetics at large applied voltages in concentrated solutions |
| url | http://hdl.handle.net/1721.1/69106 |
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