(422f) Fundamental Aspects of Concentration Polarization Arising from Non-Uniform Electrokinetic Transport

Authors: 
Khair, A. S., Carnegie Mellon University
Squires, T. M., University of California, Santa Barbara


Recent experimental studies have demonstrated that ion density gradients, or ``concentration polarization,'' can be generated from charge-selective transport through nanochannels [1], and induced-charge electro-osmosis around polarizable electrodes [2]. Remarkably, in the above examples, the large-scale concentration polarization (on the order of tens of microns) is driven by non-uniform electrokinetic transport at the nanometer scale. In this talk, we present and analyze a model system that elucidates a variety of fundamental aspects of concentration polarization. Specifically, we consider a binary symmetric electrolyte overlying a flat wall, whose surface charge varies periodically. An electric field applied parallel to the wall drives non-uniform ionic transport within the thin diffuse layer adjacent to the wall. Ions must be conserved, however. Therefore, the non-uniform transport necessitates exchange of ions between the diffuse layer and bulk electrolyte. This leads, quite naturally, to concentration polarization and bulk electric field gradients. Notably, the bulk variations are established on the macroscopic length scale of the surface charge variation, L, rather than the microscopic (Debye length, λD) thickness of the diffuse layer. Moreover, the dynamic evolution of concentration polarization occurs on the bulk diffusion time L2/D (here, D is the ion diffusivity). We formalize these ideas by deriving effective boundary conditions coupling the diffuse layer transport to the bulk electrolyte dynamics. For a weak applied field and small surface charge density (the Debye-Huckel limit), the concentration polarization is investigated for three prototypical cases: (i) a steady (DC) field; (ii) a suddenly applied field; and (iii) an oscillatory (AC) field. In particular, for a steady field we highlight the interplay of diffusion and advection in shaping the bulk concentration polarization zones.

References:

[1] S. J. Kim, Y. C. Wang, J. H. Lee, H. Jang, and J. Han, ?Concentration polarization and nonlinear electrokinetic flow near a nanofluidic channel,? Phys. Rev. Lett. 99, 044501 (2007).

[2] F. C. Leinweber, J. C. T. Eijkel, J. G. Bomer, and A. van der Berg, ?Continuous flow microfluidic demixing of electrolytes by induced charge electrokinetics in structured electrode arrays,? Anal. Chem. 78, 1425 (2006).