Electrokinetic mixing

A chip-based integrated precolumn microreactor with 1 nl reaction volume has been explored by Jacobson et al. [67]. The reactor is operated in a continuous manner by electrokinetically mixing of sample (amino acids) and reagent (o-phthaldialdehyde) streams. The reaction time is adjusted via the respective flow velocities. By switching of potentials, small plugs of the reaction product were injected into a 15.4 mm separation channel in a gated injection scheme (< 1.8% RSD in peak area). The separation efficiency achieved was relatively poor, however, electrokinetic control of reaction time (and yield) permitted to monitor the kinetics of the derivatization under pseudo first-order conditions. A similar integrated precolumn reactor operated in a stopped flow mode has been described by Harrison et al. [68]. 

Chang CC, Yang RJ (2007) Electrokinetic mixing in microfluidic systems. Microfluid Nanofluid... 

Passive Micromixers, Fig. 2 (a) Schematic of the microchip fOT parallel electrokinetic mixing, (b) Schematic of the microchip for serial electrokinetic mixing [4]... 

In another study, a microchip-based electrochemical enzyme immunoassay was developed by Chatrathi and colleagues [65] and its performance is demonstrated for the determination of monoclonal mouse IgG as a model analyte. The direct homogeneous immunoassay requires the integration of electrokinetic mixing of ALP-labeled anti-mouse IgG antibody (Ab-E) with the mouse IgG antigen (Ag) analyte in a precolumn reaction chamber, injection of immunochemical products into the separation channel, followed by rapid electrophoretic separation of enzyme-labeled free antibody and enzyme-labeled antibody-antigen complex. The separation is followed by a postcolumn reaction of enzyme tracer with p-aminophenyl... 

R., Khaledi, M. G. Quantitative structure-activity relationships studies with micellar electrokinetic chromatography. Influence of surfactant type and mixed micelles on estimation of hydrophobicity and bioavailability. J. Chromatogr. A 1996, 727, 323-335. 

Clothier, Jr., J. G. and Tomellini, S. A., Chiral separation of veraprimil and related componds using micellar electrokinetic capillary chromatography with mixed micells of bile salt and polyoxyethylene ethers,. Chromatogr. A, 712, 179,1996. 

Thomas, B. R., and Ghodbane, S. (1993). Evaluation of a mixed micellar electrokinetic capillary electrophoresis method for validated pharmaceutical quality-control. /. Liq. Chromatogr. 16, 1983-2006. 

Dehouck, P., Van Schepdael, A., Roets, E., and Hoogmartens, J. (2001). Analysis of clindamycin by micellar electrokinetic chromatography with a mixed micellar system. /. Chromatogr. A 932, 145-152. 

Kutter, J. P., Jacobson, S. C., and Ramsey, J. M. (1997). Integrated microchip device with electrokinetically controlled solvent mixing for isocratic and gradient elution in micellar electrokinetic chromatography. Anal. Chem. 69, 5165-5171. 

M Hong, BS Weekley, SJ Grieb, JP Foley. Electrokinetic chromatography using thermodynamically stable vesicles and mixed micelles formed from oppositely charged surfactants. Anal. Chem. 70 1394-1403 (1998). 

Sarahney et al. (2005) investigated the application of an ex situ electrokinetic process to remove arsenic, copper, and chromium from CCA-treated wood chips. According to Sarahney et al. (2005), oxalic acid and oxalic acid mixed with EDTA enhanced the treatment process for arsenic. The oxalic acid and EDTA mixture removed 88 % of the arsenic from the wood. 

The impact of electric current on the environmental conditions and the anaerobic microbial activity in completely mixed fed-batch reactors was studied at various electric field strengths (Maillacheruvu and Alshawabkeh, 1999). Experiments were conducted in bio-electrokinetic (BioEK.) reactors, which consist of plexiglass boxes with titanium-coated mesh electrodes mounted at both ends. Electric fields of 1.5 V/cm through 6 V/cm were applied. Unacclimated anaerobic cultures obtained from a mesophilic anaerobic digester were used in these experiments. 

S. C. Jacobson, T. E. McKnight, and J. M. Ramsey, Microlluidic devices for electrokinetically driven parallel and serial mixing, Anal. Chem. 71, 4455-4459 (1999). 

External energy sources for active mixing are, for example, ultrasound [22], acoustic, bubble-induced vibrations [23,24], electrokinetic instabilities [25], periodic variation of flow rate [26-28], electrowetting induced merging of droplets [29], piezoelectric vibrating membranes [30], magneto-hydrodynamic action [31], small impellers [32], integrated micro valves/pumps [33] and many others, which are listed in detail in Section 1.2. 

Electric fields may interact with flows fed by hydrostatic or pumping action [91]. Flows driven by electroosmotic means may be mixed as well by the action of fluctuating electric fields, which creates oscillating electroosmotic flows and has been termed electrokinetic instability (EKI) [25, 93], In this way, rapid stretching and folding of material lines are induced, not unlike the effect of stirring. In one realized example, comparatively low frequencies, below -100 Hz, and electric field strengths in excess of 100 V mm1 are applied for channels with dimensions of about 50 pm [25],... 

Figure 1.8 Design of an electrokinetic instability micro mixer, second-generation device, based on the results obtained with the first design given in Figure 1.7. The electrokinetic instability is confined to the square mixing chamber shown in the center of the schematic and, to a small extent, to fluid channel regions attached to it [25] (by courtesy of ACS).

This electrokinetically driven micro mixer uses localized capacitance effects to induce zeta potential variations along the surface of silica-based micro channels [92], The zeta potential variations are given near the electrical double layer region of the electroosmotic flow utilized for species transport. Shielded ( buried ) electrodes are placed underneath the channel structures for the fluid flow in separate channels, i.e. they are not exposed to the liquid. The potential variations induce flow velocity changes in the fluid and thus promote mixing [92],... 

Dielectrophoresis is the translational motion of neutral matter owing to polarization effects in a non-uniform electric field. Depending on matter or electric parameters, different particle populations can exhibit different behavior, e.g. following attractive or repulsive forces. DEP can be used for mixing of charged or polarizable particles by electrokinetic forces [48], In particular, dielectric particles are mixed by dielectrophoretic forces induced by AC electric fields, which are periodically switched on and off. 

Figure 1.14 Two-dimensional power spectra of various mixing chamber images for the electrokinetic instability micro mixer, second-generation device, (a) Large frequency components along the vertical direction owing to the initial layered distribution of the dye. (b) Larger spatial frequencies are introduced by the EKI stirring within the chamber, (c) The attenuation of large spatial frequencies corresponds to a nearly homogeneous intensity profile [25] (by courtesy of ACS).

Concentration profiles were derived by gray-scale analysis from the images mentioned above [92], Whereas without use of zeta potential variation no difference in concentration profiles between the upstream and downstream positions is visible, a much more flattened, i.e. mixed, profile results under electrokinetically driven conditions in the downstream position. 

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