Electrokinetic flow-induced currents in silica nanofluidic channels

333

306

In an aqueous solution, the surface of inorganic nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions. In the presence of a concentration gradient, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of one type of ions. The Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using inorganic ion-selective nanochannels. Silica nanochannels with heights of 4, 26, and 80 nm were used in this study. We experimentally investigated the power generation from these nanochannels placed between two potassium chloride solutions with various combinations of concentrations. The power generation per unit channel volume increases when the concentration gradient increases, and also increases as channel height decreases. The highest power density measured was 7.7 W/m 2 . Our data also indicate that the energy conversion efficiency and the ion selectivity increase with a decrease of concentrations and channel height. The best efficiency obtained was 31%. Power generation from concentration gradients in inorganic ion-selective nanochannels could be used in a variety of applications, including micro batteries and micro power generators.

Section: Resultsmentioning

confidence: 99%

Power generation from concentration gradient by reverse electrodialysis in ion-selective nanochannels

Kim

Duan

Chen

et al. 2010

333

306

“…In nanochannels, there is an overlap of charge-shielding layers, which is called electric double layers (EDLs) (Kim et al 2007. CP affects ion perm-selectivity, conductivity, and electric field profiles around micro/nanochannel interfaces as it changes ion transport (Mani et al 2009;Choi and Kim 2009;Plecis et al 2008). A further increased electric field results in the breakup of a CP structure, generating vortices near micro/ nanochannel interfaces (Yossifon and Chang 2008).…”

Section: Introductionmentioning

confidence: 99%

Millisecond-order rapid micromixing with non-equilibrium electrokinetic phenomena

Lee

Kim

2011

We report an active micromixer utilizing vortex generation due to non-equilibrium electrokinetics near micro/nanochannel interfaces. Its design is relatively simple, consisting of a U-shaped microchannel and a set of nanochannels. We fabricated the micromixer just using a two-step reactive ion etching process. We observed strong vortex generation in fluorescent microscopy experiments. The mixing performance was evident in a combined pressure-driven and electroosmotic flows, compared with the case with a pure pressure-driven flow. We characterized the micromixer for several conditions: different applied voltages, ion concentrations, flow rates, and nanochannel widths. The experimental results show that the mixing performance is better with a higher applied voltage, a lower ion concentration, and a wider nanochannel width. We quantified the mixing characteristics in terms of mixing time. The lowest mixing time was 2 milliseconds with the voltage of 230 V and potassium chloride solutions of 0.1 mM. We expect that the micromixer is beneficial in several applications requiring rapid mixing.Keywords Micromixers Á Rapid mixing Á Electroosmosis of the second kind Á Non-equilibrium electrokinetics Á Nanochannels

“…More realistic is the so-called charge-regulation model, which takes into account the fact that an increase in the surface potential gives rise to a stronger electrostatic adsorption of potential-determining ions (for example, H ? ) and, consequently, a decrease in the density of surface charge due to the smaller extent of dissociation of ionogenic groups (Chang and Yang 2009;Choi and Kim 2009;Hung et al 2008;Jiang and Stein 2010). This decrease can be quantified in a simple way:…”

Section: Quantification Of Nano-channel Criterion By Means Of Capillamentioning

confidence: 99%

What makes a nano-channel? A limiting-current criterion

Yaroshchuk

2011

It is shown theoretically that the occurrence of limiting currents at micro-/nano-interfaces critically depends on the ratio of nano-channel height (nano-pore size) and the Debye screening length. If this ratio is sufficiently large ([ca.6 for slit-like nano-channels, for example), the limiting-current phenomenon is suppressed due to the electroosmotic transfer of salt toward the polarized micro-/nano-interface. Therefore, not too narrow nanochannels effectively behave like micro-channels in the limiting-current terms. It is suggested that this qualitative change in the behavior can be used as a signature of ''genuine'' nano-channels. It is also shown that some recent experimental findings on the concentration polarization of joined micro-/nano-fluidic systems can be explained by using this criterion. List of symbols a s Hydrodynamic radius of salt ''molecule'' (m) c Virtual salt concentration (mole/m 3 ) c ± Concentrations of cations and anions in virtual solution (mole/m 3 ) c i Salt concentration at the micro-/nanointerface (mole/m 3 ) c 0 Salt concentration in reservoirs (mole/m 3 ) D s Salt diffusion coefficient in bulk solution (m 2 /s) D ± Diffusion coefficients of ions in bulk solution (m 2 /s) F Faraday constant (C/mole) g Electric conductivity of nano-block at zero pressure difference (S/m) I Current density (A/m 2 ) I lim Limiting-current density (A/m 2 ) j ± Ion fluxes (mole/m 2 Ás) J v Volume flow per unit area (m/s) J s Effective salt flux (mole/m 2 Ás) L m Length of micro-channel (m) m 3a s r B Levine constant (-) P ± Ionic permeabilities (m 2 /s) P s diffusion permeability to salt (m 2 /s) P e Péclet number (-) R Universal gas constant (Joule/K) r B Bjerrum radius (m) r p Pore radius (m) T Absolute temperature (K) t ? (m) Cation transport number in the membrane (-) t ? (n) Cation transport number in the nanoblock (-) t ? (b) Cation transport number in the solution (-) Dt þ t n ð Þ þ À t b ð Þ þ Difference of cation transport number between nano-block and solution (-) Z ? Ion charges (in proton-charge units) (-) Electronic supplementary material The online version of this article (