Electroosmotic micro-flow

For the Michael addition of 2,4-pentanedione enolate to ethyl propiolate, improvements in conversion were determined. This example serves also to demonstrate that proper process conditions are mandatory to have success with micro-reactor processing. A conversion of only 56% was achieved when using electroosmotically driven flow with a two-fold injection, the first for forming the enolate and the second for its addition to the triple bond (batch synthesis 89%) [151]. Using instead a stopped-flow technique to enhance mixing, a conversion of 95% was determined. 

OS 44] [R 4a] [P 33] By switching on and off the electroosmotic-driven flow of one reactant, plugs can be inserted in the reaction channel [6]. 4-Bromobenzonitrile plugs (5 s) are inserted into a continuous phenylboronic acid stream. By this means, the effective concentration of the aryl halide in the micro channel can be increased. 

Haswell, S.J., Development and operating characteristics of micro flow injection analysis systems based on electroosmotic flow. Analyst 1997, 122, 1R-10R. 

F.-Q. Nie, M. Macka, B. Pauli, Micro-flow injection analysis system on-chip sample preconcentration, injection and delivery using coupled monolithic electroosmotic pumps, Lab Chip 7 (2007) 1597. 

In most microfluidics and nanofluidics, the atomistic effects on electroosmotic flows are neghgible. But when the characteristic length is comparable with the molecular size of fluid, it should be considered. Molecular dynamics methods have been used to simulate the particle effects in nanoscale electroosmotic flows [23, 24]. However, it is too time-consuming to simulate a real electroosmotic micro- and nanofluidics by molecular dynamics. The multiscale modeling and analysis would be a possible research direction. 

Liquid transport is achieved by hydrostatic action, pumping or electroosmotic flow (EOF). So far, chip reactors have been employed at low to very low flow rates, e.g. from 1 ml min to 1 pi min. Applications consequently were restricted to the laboratory-scale or even solely to analytics. However, this is not intrinsic. By choosing larger internal dimensions, similar throughputs as for the other classes of liquid or liquid/liquid micro reactors are in principle achievable. 

For electroosmotic flow transport, a tube was inserted into the base plate, connected to the micro channel [19],... 

P 12] The micro channels were primed with anhydrous N,N-dimethylformamide (DMF) to remove air and moisture before carrying out the reaction [88]. A 50 pi volume of a solution of Fmoc-y9-alanine (0.1 M) in anhydrous DMF was placed in one reservoir of a micro chip, driven by electroosmotic flow. A 50 pi volume of a solution of EDCl (0.1 M) in anhydrous DMF and 50 pi of a solution of Dmab-y9-alanine (0.1 M) in anhydrous DMF were inserted in the other two reservoirs. Anhydrous DMF was placed in the fourth reservoir, for collection of the product. Room temperature was applied for reaction for a 20 min period. The voltage was set in the range 500-700 V. 

P 13] The micro channels were primed with anhydrous DMF to remove air and moisture before carrying out the reaction [5, 88]. A 50 pi volume of a solution of the pentafluorophenyl ester of Fmoc-y9-alanine (0.1 M) in anhydrous DMF was placed in one reservoir of a micro chip, driven by electroosmotic flow (Figure 4.42). 

OS 16] ]R 5] ]P 13] Using continuous flow in an electroosmotic-driven micro reactor gave a quantitative yield of the dipeptide in only 20 min (600 V for Dmab-/ -alanine 700 V for the Fmoc ester) [5, 88]. Batch synthesis under the same conditions gave only a 40-50% yield [5] (46% in [5]), needing 24 h. 

In addition to the general improvement of transfer in micro reactors, there is evidence that the voltage of electroosmotic flow (for EOF see [14]) in combination with the large internal surface area in glass chips can induce hydroxide ion formation [6]. Concerning catalyst loss, there is no obvious direct correlation rather, micro reactors can act as mini fixed beds fixing heterogeneous catalyst particles. 

P 38] Ethanol solutions of ethyl propiolate and diisopropylethylamine were pumped via electroosmotic flow through the micro channels of the reactor [8], By mixing thereof the enolate was obtained. By subsequent contacting with the 1,3-dicarbonyl compound, the product was obtained. The temperature was set to room temperature. In a period of 20 min a volume sufficiently large for analysis was sampled. The reaction product spectra was analyzed by GC/MS via comparison with synthetic standards. The remaining amount of diketone was used for calculating conversions. 

The aim of one study was to show that arrays of cycloadducts, from various precursors, can be made in a single run in one chip [18]. In addition, this study served more generally to demonstrate the feasibility and advantages of pressure-driven flow in micro chips exemplary of one prominent organic reaction. The advantages and drawbacks of pressure-driven flow as compared with electroosmotic flow (for EOF see [14]) were discussed [18]. 

Wilson, N. G., McGreedy, T, Microporous silica structures for the immobilization of catalysts and enhancement of electroosmotic flow (EOF) in micro reactors, in Ehreeld, W. (Ed.), Microreaction Technology 3rd International Conference on Microreaction Technology, Proc. of IMRET 3,... 

Nikbin N, Watts P (2004) Solid-supported continuous flow synthesis in micro-reactors using electroosmotic flow. Org Process Res Dev 8 942 Overbeek JTG (1952) Electro chemistry of double layer. In Kruyt HR (ed) Colloid science, Vol. 1. Elsevier, Amsterdam... 

Mixer 3 [M 3] Electrokinetic Instability Electroosmotic Flow Micro Mixer, First-generation Device... 

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],... 

Figure 1.17 Schematic of a micro channel equipped with many electrodes at the upper (U ) and lower (Lf) walls for control of the C, potential. The arrows in the channel denote the directions of the electroosmotic velocities creating one type of flow pattern, here a counter-current arrangement of top and bottom flows (top) alternating-flow arrangement, demonstrating another type of control over the potential (bottom) [28] (by courtesy of ACS).

A micro channel of height 2 H is equipped with electrodes at the upper (L/,) and lower (L walls [28], These electrodes are used to control the C, potential at the solid-liquid interface. In this way, the direction of the electroosmotic flow near the interface can be changed locally. The external electric field is given as Ex. 

Kirby, B.J., Wheeler, A.R., Shepodd, T.J., Fruetel, J.A., Hasselbrink, E.F., Zare, R.N., A laser-polymerized thin film silica surface modification for suppression of cell adhesion and electroosmotic flow in microchannels. Micro Total Analysis Systems, Proceedings 5th pTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 605-606. 

In most electroosmotic flows in microchannels, the flow rates are very small (e.g., 0.1 pL/min.) and the size of the microchannels is very small (e.g., 10 100 jm), it is extremely difficult to measure directly the flow rate or velocity of the electroosmotic flow in microchannels. To study liquid flow in microchannels, various microflow visualization methods have evolved. Micro particle image velocimetry (microPIV) is a method that was adapted from well-developed PIV techniques for flows in macro-sized systems [18-22]. In the microPIV technique, the fluid motion is inferred from the motion of sub-micron tracer particles. To eliminate the effect of Brownian motion, temporal or spatial averaging must be employed. Particle affinities for other particles, channel walls, and free surfaces must also be considered. In electrokinetic flows, the electrophoretic motion of the tracer particles (relative to the bulk flow) is an additional consideration that must be taken. These are the disadvantages of the microPIV technique. 

G.N. Doku, S.J. Haswell, Further studies into the development of a micro-FIA (pFIA) system based on electroosmotic flow for the determination of phosphate as orthophosphate, Anal. Chim. Acta 382 (1999) 1. 

Combined Pressure-Driven Flow and Electroosmotic Flow - Control of Micro-Fluidics... 

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