Electroosmotic flow manipulation

Electroosmotic flow (EOE) is thus the mechanism by which liquids are moved from one end of the sepai ation capillai y to the other, obviating the need for mechanical pumps and valves. This makes this technique very amenable to miniaturization, as it is fai simpler to make an electrical contact to a chip via a wire immersed in a reservoir than to make a robust connection to a pump. More important, however, is that all the basic fluidic manipulations that a chemist requires for microchip electrophoresis, or any other liquid handling for that matter, have been adapted to electrokinetic microfluidic chips. 

Vaidya, B., Soper, S.A., McCarley, R.L., Surface modification and characterization of microfabricated polycarbonate) devices manipulation of electroosmotic flow. Analyst, 2002, 127, 1289-1292. 

The key operational parameter in free-solution capillary electrophoresis is the pH of the running buffer, as the electroosmotic flow and ionization of the analyte can be regulated by this variable. The role of buffers in capillary electrophoresis has been discussed in detail, with emphasis on buffer concentration, buffer type, and pH effects [10]. The effect of organic solvents on separation and migration behavior has been studied for dipeptides [11] and somatostatin analog peptides [12]. The order of migration as well as the selectivity may be manipulated by organic modifiers in... 

Another approach to fraction collection is the use of an on-column frit structure or capillary fracture that depends on the electroosmotic flow to deposit the eluent in a continuous manner on a moving surface. Although this approach circumvents the dilution problem, the collection structures are complex and can result in the loss of some of the analyte. One commercially available fraction collection device couples CE with membrane fraction collection, without the need for frits or capillary fractures. The outlet vial holder can be removed and replaced with a wetted circular polyvinylidene difluoride (PVDF) disk, which enables the collection of eluted analytes and subsequent manipulations such as immunoblotting and microsequencing. Figure 6.13 shows a schematic diagram of the CE membrane fraction collector interface.74... 

Various active micromixers using 2-D time-dependent flow to achieve chaotic advection have been developed [16-19]. Since electroosmosis is very attractive for manipulating fluids in LOC devices, a chaotic electroosmotic stirrer developed by Qian and Bau [20] is described as an example to achieve chaotic advection and mixing by 2-D time-dependent electroosmotic flow. 

In the late 1990s, Ramos et al. discovered steady electroosmotic flow over a pair of microelectrodes applying an AC voltage and dubbed the effect AC electroosmosis [1]. Around the same time, Ajdari predicted ACEO flow over periodic electrode arrays and showed how the effect could be used for long-range pumping [2]. As the performance of ACEO pumps has advanced [3, 4], ACEO has also been exploited, in conjunction with dielectrophoresis (DEP), in different geometries to manipulate particles and cells in microfluidic devices [5-7]. 

A myriad of conditions could give rise to nonuniform charge distributions in the fluid. For example, electroosmotic flows or solutions of electrolytes are often used to manipulate microfluidic flows. In these situations, the macromolecule may be subject to Coulomb forces. There is no general expression to account for these forces, but they will depend on the location and strength of the charge distribution in the fluid as well as along the molecule. 

As mentioned above we have recently developed a novel on-chip electrokinetically-based microfluidic technique for SNP discrimination (see Erickson et al. [12] for details). The traditional advantages in terms of flow and sample handling afforded by electrokinetic transport described in the electroosmotic flow section of the article were extended to include thermal control through on-line manipulation of the Joule heating conditions. Precise control of the thermal, shear and electrical conditions at the... 

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