Deionization shocks in microstructures. A. Mani and M.Z. Bazant. Physical Review E, 84(6), 2011. (URL)
Salt transport in bulk electrolytes is limited by diffusion and advection, but in microstructures with charged surfaces (e.g. microfluidic devices, porous media, soils, or biological tissues) surface conduction and electro-osmotic flow also contribute to ionic fluxes. For small applied voltages, these effects lead to well known linear electrokinetic phenomena. In this paper, we predict some surprising nonlinear dynamics that can result from the competition between bulk and interfacial transport at higher voltages. When counter-ions are selectively removed by a membrane or electrode, a ``deionization shock'' can propagate through the microstructure, leaving in its wake an ultrapure solution, nearly devoid of co-ions and colloidal impurities. We elucidate the basic physics of deionization shocks and develop a mathematical theory of their existence, structure, and stability, allowing for slow variations in surface charge or channel geometry. Via asymptotic approximations and similarity solutions, we show that deionization shocks accelerate and sharpen in narrowing channels, while they decelerate and weaken, and sometimes disappear, in widening channels. These phenomena may find applications in separations (deionization, decontamination, biological assays) and energy storage (batteries, supercapacitors) involving electrolytes in microstructures.
@ARTICLE { mani_bazant_2011,
ABSTRACT = { Salt transport in bulk electrolytes is limited by diffusion and advection, but in microstructures with charged surfaces (e.g. microfluidic devices, porous media, soils, or biological tissues) surface conduction and electro-osmotic flow also contribute to ionic fluxes. For small applied voltages, these effects lead to well known linear electrokinetic phenomena. In this paper, we predict some surprising nonlinear dynamics that can result from the competition between bulk and interfacial transport at higher voltages. When counter-ions are selectively removed by a membrane or electrode, a ``deionization shock'' can propagate through the microstructure, leaving in its wake an ultrapure solution, nearly devoid of co-ions and colloidal impurities. We elucidate the basic physics of deionization shocks and develop a mathematical theory of their existence, structure, and stability, allowing for slow variations in surface charge or channel geometry. Via asymptotic approximations and similarity solutions, we show that deionization shocks accelerate and sharpen in narrowing channels, while they decelerate and weaken, and sometimes disappear, in widening channels. These phenomena may find applications in separations (deionization, decontamination, biological assays) and energy storage (batteries, supercapacitors) involving electrolytes in microstructures. },
AUTHOR = { A. Mani and M.Z. Bazant },
JOURNAL = { Physical Review E },
TITLE = { Deionization shocks in microstructures },
URL = { http://dx.doi.org/10.1103/PhysRevE.84.061504 },
YEAR = { 2011 },
VOLUME = { 84 },
NUMBER = { 6 },
1 = { http://dx.doi.org/10.1103/PhysRevE.84.061504 },
}
Mani Research Group Mechanical Engineering Dept., Stanford University 488 Escondido Mall, Building 500 Room 500M Stanford, CA 94305-3024, USA |
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