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Understanding electrokinetics at the nanoscale: A perspective
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1.
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http://aip.metastore.ingenta.com/content/aip/journal/bmf/3/1/10.1063/1.3056045
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Figures

Image of FIG. 1.

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FIG. 1.

Nonlinear dc I-V curve across a nanoslit at different electrolyte strengths. The chip is formed by anodic bonding of the two Pyrex glass slides depicted in the inset. The resulting nanoslit dimensions are width , height , and length , while the two microreservoirs at the end of the nanoslit are squares of about in length and in depth.

Image of FIG. 2.

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FIG. 2.

Cyclic voltametry across a similar nanoslit to that of Fig. 1 at different scan rates.

Image of FIG. 3.

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FIG. 3.

(a) Magnitude and (b) phase of the ac impedance of the nanoslit in Fig. 1 with different dc biases at an electrolyte strength of .

Image of FIG. 4.

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FIG. 4.

Nanoslit conductance as a function of the electrolyte concentration (symbols—experiment; continuous line—model (1); dashed line—bulk conductance). The nanoslit (schematically described in the inset) dimensions are width , height , and length .

Image of FIG. 5.

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FIG. 5.

Extended polarized layer at the surface of a membrane. The spatial coordinate has been normalized by the thickness of the diffusion layer, which is arbitrarily imposed. An electroneutral diffusion layer region is seen on the left with a linear concentration profile (a) and a logarithm potential drop (b). Within it lies an extended polarized layer with an excess of cations, which are the counterions. A near equilibrium Poisson–Boltzmann region is observed next to the membrane. The external diffusion layer can extend to the electrode or can terminate a short distance from the membrane (Reprinted with permission—Ref. 16).

Image of FIG. 6.

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FIG. 6.

Schematics of Rubinstein’s instability physical mechanism: (a) a small perturbation of the extended polarized layer thickness (increased thickness at the center) resulting in transverse Maxwell pressure gradients that lead to a vortex pair in a positive feedback loop; (b) concentration profiles (normalized by the bulk concentration) of the different ionic species; and (c) electric potential profile (normalized by the applied voltage) across the center (blue line) and off-center (red line) of the perturbation region.

Image of FIG. 7.

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FIG. 7.

Rubinstein’s vortex instability in front of the nanoslit in Fig. 4 develops beyond a critical voltage to select a thin diffusion layer much shorter than the distance to the electrode. (a) The confocal images are taken at showing vortices at the depletion layer on the anodic side (left reservoir) for an ionic strength of . The diffusion layer is allowed to grow to a certain thickness with a low-frequency ac field and the vortices only appear when the frequency is below and when the voltage is beyond (enhanced online). (b) A Log-Log graph of the maximum extent of the experimentally measured vs the frequency. A clear break from the diffusive scaling, (continuous line), occurs at a frequency smaller than corresponding to the instability selected length scale. (c) The thickness of the instability selected diffusion layer is recorded for each voltage. Below , the diffusion layer essentially extends to the electrode (Reprinted with permission—Ref. 18). [URL: http://dx.doi.org/10.1063/1.3056045.1]10.1063/1.3056045.1

Image of FIG. 8.

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FIG. 8.

Rectification of a nanoslit with asymmetric entrances. The left entrance is wider than the right , as seen in the microscopic image (inset of (b)), and is obtained by intentional misalignment of the two Pyrex slides. The nanochannel height is and length is . (a)I-V measurements for varying ionic strengths and voltage bias (continuous line, anode at the right reservoir—reverse bias; dashed lines, anode at the left reservoir—forward bias); (b) the rectification factor defined as the ratio between the forward and reverse biases; (c) the depletion-enrichment phenomenon when the anode is at the right reservoir (enhanced online). [URL: http://dx.doi.org/10.1063/1.3056045.2]10.1063/1.3056045.2

Image of FIG. 9.

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FIG. 9.

Communication between neighboring nanochannels due to overlapping polarized layers at the entrance. (a) The current for a seven-channel array (solid) is compared to seven times that for a single-channel array (dashed) to determine the degree of communication (Reprinted with permission—Ref. 22). The single nanochannel dimensions are width , height , and length . (b) An array of separate depletion regions emerging from the nanochannel array, just before their merging (enhanced online). [URL: http://dx.doi.org/10.1063/1.3056045.3]10.1063/1.3056045.3

Image of FIG. 10.

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FIG. 10.

Nanocolloids that assemble and disassemble within the nanochannel (width and depth ) into a colloidal closed-packed structure (enhanced online). [URL: http://dx.doi.org/10.1063/1.3056045.4]10.1063/1.3056045.4

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/content/aip/journal/bmf/3/1/10.1063/1.3056045
2009-01-02
2014-04-21

Abstract

Electrokinetics promises to be the microfluidic technique of choice for portable diagnostic chips and for nanofluidic molecular detectors. However, despite two centuries of research, our understanding of ion transport and electro-osmotic flow in and near nanoporousmembranes, whose pores are natural nanochannels, remains woefully inadequate. This short exposition reviews the various ion-flux and hydrodynamic anomalies and speculates on their potential applications, particularly in the area of molecular sensing. In the process, we revisit several old disciplines, with some unsolved open questions, and we hope to create a new one.

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Scitation: Understanding electrokinetics at the nanoscale: A perspective
http://aip.metastore.ingenta.com/content/aip/journal/bmf/3/1/10.1063/1.3056045
10.1063/1.3056045
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