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Electroosmotic flow in nanofluidic channels.

Haywood DG, Harms ZD, Jacobson SC - Anal. Chem. (2014)

Bottom Line: We report the measurement of electroosmotic mobilities in nanofluidic channels with rectangular cross sections and compare our results with theory.However, for κh < 10, the electrical double layer extends into the nanochannels, and due to confinement within the channels, the average electroosmotic mobilities decrease.At κh ≈ 4, the electroosmotic mobilities in the 27, 54, and 108 nm channels exhibit maxima, and at 0.1 mM NaCl, the electroosmotic mobility in the 27 nm channel (κh = 1) is 5-fold lower than the electroosmotic mobility in the 2.5 μm channel (κh = 100).

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Indiana University , 800 E. Kirkwood Ave., Bloomington, Indiana 47405-7102, United States.

ABSTRACT
We report the measurement of electroosmotic mobilities in nanofluidic channels with rectangular cross sections and compare our results with theory. Nanofluidic channels were milled directly into borosilicate glass between two closely spaced microchannels with a focused ion beam instrument, and the nanochannels had half-depths (h) of 27, 54, and 108 nm and the same half-width of 265 nm. We measured electroosmotic mobilities in NaCl solutions from 0.1 to 500 mM that have Debye lengths (κ(-1)) from 30 to 0.4 nm, respectively. The experimental electroosmotic mobilities compare quantitatively to mobilities calculated from a nonlinear solution of the Poisson-Boltzmann equation for channels with a parallel-plate geometry. For the calculations, ζ-potentials measured in a microchannel with a half-depth of 2.5 μm are used and range from -6 to -73 mV for 500 to 0.1 mM NaCl, respectively. For κh > 50, the Smoluchowski equation accurately predicts electroosmotic mobilities in the nanochannels. However, for κh < 10, the electrical double layer extends into the nanochannels, and due to confinement within the channels, the average electroosmotic mobilities decrease. At κh ≈ 4, the electroosmotic mobilities in the 27, 54, and 108 nm channels exhibit maxima, and at 0.1 mM NaCl, the electroosmotic mobility in the 27 nm channel (κh = 1) is 5-fold lower than the electroosmotic mobility in the 2.5 μm channel (κh = 100).

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Variation of channelconductivity with bulk conductivity and NaClconcentration. Channel half-depths (h) are 27 nm,54 nm, 108 nm, and 2.5 μm. Lines for each channel half-depthare calculated with eq 1 from the specific surfaceconductivities, bulk conductivities, and channel dimensions. Measurementswere made on three devices with h = 27 nm, four deviceswith h = 54 nm, and two devices with h = 108 nm and 2.5 μm. Error bars are ± σ.
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fig2: Variation of channelconductivity with bulk conductivity and NaClconcentration. Channel half-depths (h) are 27 nm,54 nm, 108 nm, and 2.5 μm. Lines for each channel half-depthare calculated with eq 1 from the specific surfaceconductivities, bulk conductivities, and channel dimensions. Measurementswere made on three devices with h = 27 nm, four deviceswith h = 54 nm, and two devices with h = 108 nm and 2.5 μm. Error bars are ± σ.

Mentions: Figure 2 shows the variation of the channel conductivity with NaCl concentrationand bulk solution conductivity. As expected, at high salt concentrations,the nanochannel conductivities match the bulk solution conductivities.However, at low salt concentrations, the nanochannel conductivitiesdeviate from linearity and are significantly higher than the conductivitiesin the microchannel and bulk solution.18,21,59 The deviation increases as the channel half-depthdecreases, i.e., the shallowest nanochannel (h =27 nm) has the highest conductivities for NaCl concentrations of 0.1,1, and 10 mM. Deviation from linearity occurs because a significantportion of the current is carried through the nanochannel by surfacecharge. As the surface-to-volume ratio of the channel increases, surfacecharge contributes a much greater fraction of current transported,which results in higher channel conductivities.


Electroosmotic flow in nanofluidic channels.

Haywood DG, Harms ZD, Jacobson SC - Anal. Chem. (2014)

Variation of channelconductivity with bulk conductivity and NaClconcentration. Channel half-depths (h) are 27 nm,54 nm, 108 nm, and 2.5 μm. Lines for each channel half-depthare calculated with eq 1 from the specific surfaceconductivities, bulk conductivities, and channel dimensions. Measurementswere made on three devices with h = 27 nm, four deviceswith h = 54 nm, and two devices with h = 108 nm and 2.5 μm. Error bars are ± σ.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4238593&req=5

fig2: Variation of channelconductivity with bulk conductivity and NaClconcentration. Channel half-depths (h) are 27 nm,54 nm, 108 nm, and 2.5 μm. Lines for each channel half-depthare calculated with eq 1 from the specific surfaceconductivities, bulk conductivities, and channel dimensions. Measurementswere made on three devices with h = 27 nm, four deviceswith h = 54 nm, and two devices with h = 108 nm and 2.5 μm. Error bars are ± σ.
Mentions: Figure 2 shows the variation of the channel conductivity with NaCl concentrationand bulk solution conductivity. As expected, at high salt concentrations,the nanochannel conductivities match the bulk solution conductivities.However, at low salt concentrations, the nanochannel conductivitiesdeviate from linearity and are significantly higher than the conductivitiesin the microchannel and bulk solution.18,21,59 The deviation increases as the channel half-depthdecreases, i.e., the shallowest nanochannel (h =27 nm) has the highest conductivities for NaCl concentrations of 0.1,1, and 10 mM. Deviation from linearity occurs because a significantportion of the current is carried through the nanochannel by surfacecharge. As the surface-to-volume ratio of the channel increases, surfacecharge contributes a much greater fraction of current transported,which results in higher channel conductivities.

Bottom Line: We report the measurement of electroosmotic mobilities in nanofluidic channels with rectangular cross sections and compare our results with theory.However, for κh < 10, the electrical double layer extends into the nanochannels, and due to confinement within the channels, the average electroosmotic mobilities decrease.At κh ≈ 4, the electroosmotic mobilities in the 27, 54, and 108 nm channels exhibit maxima, and at 0.1 mM NaCl, the electroosmotic mobility in the 27 nm channel (κh = 1) is 5-fold lower than the electroosmotic mobility in the 2.5 μm channel (κh = 100).

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Indiana University , 800 E. Kirkwood Ave., Bloomington, Indiana 47405-7102, United States.

ABSTRACT
We report the measurement of electroosmotic mobilities in nanofluidic channels with rectangular cross sections and compare our results with theory. Nanofluidic channels were milled directly into borosilicate glass between two closely spaced microchannels with a focused ion beam instrument, and the nanochannels had half-depths (h) of 27, 54, and 108 nm and the same half-width of 265 nm. We measured electroosmotic mobilities in NaCl solutions from 0.1 to 500 mM that have Debye lengths (κ(-1)) from 30 to 0.4 nm, respectively. The experimental electroosmotic mobilities compare quantitatively to mobilities calculated from a nonlinear solution of the Poisson-Boltzmann equation for channels with a parallel-plate geometry. For the calculations, ζ-potentials measured in a microchannel with a half-depth of 2.5 μm are used and range from -6 to -73 mV for 500 to 0.1 mM NaCl, respectively. For κh > 50, the Smoluchowski equation accurately predicts electroosmotic mobilities in the nanochannels. However, for κh < 10, the electrical double layer extends into the nanochannels, and due to confinement within the channels, the average electroosmotic mobilities decrease. At κh ≈ 4, the electroosmotic mobilities in the 27, 54, and 108 nm channels exhibit maxima, and at 0.1 mM NaCl, the electroosmotic mobility in the 27 nm channel (κh = 1) is 5-fold lower than the electroosmotic mobility in the 2.5 μm channel (κh = 100).

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