Interdisciplinary Applied Mathematics

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Joule heating has adverse effects on microfluidic device performance. For example, local temperature increases result in local reductions in the absolute viscosity of the fluid. If we neglect temperature-dependent variations of any other properties, then the local viscosity reductions lead to increased electroosmotic slip velocity. For constant volumetric flowrate such local variations are compensated by onset of local pressure-driven flow. Therefore, under substantial Joule heating, it will not be possible to maintain pluglike velocity distribution in electroosmotic flows. This adversely affects the species transport, and it results in enhanced dispersion (see Section 7.5.3). However, we must note that miniaturization of device components reduces Joule heating and its adverse effects for the following reasons:


First, large electric field gradients can be achieved with relatively smaller potential differences between the electrodes; hence ||E|| is reduced.


Second, reduction in volume reduces the total heat generation.


Third, increased surface area to volume ratio enhances heat loss to the environment through heat conduction.

7.4-7 Applications


In this section, we present microfluidic applications of electroosmotic flows. We first present suppression of electroosmosis, which becomes important in certain applications. Then we present mixing enhancement with electroosmotic flows, followed by electroosmotic flow control examples.


Suppression of Electroosmosis with Zeta Potential Modifications


Although electroosmosis is an attractive technique for microfluidic pumping, it may need to be suppressed or modified for certain applications, such as the capillary isoelectric focusing (IEF) and on-chip IEF, by altering the zeta potential. This can be obtained by various techniques, including polymer coatings and embedded surface electrodes. There are two basic polymer coating techniques: static and dynamic coating. The static coating is based on covalent bonding between the coating material and capillary surface, but dynamic coating relies on ionic interactions (Horvath and Dolnik, 2001; Righetti et al., 2001). Dynamic coating is an active field of research in chemistry. (Liu et al., 2000) utilized different polymer bilayers, such as a cationic layer of polybrene and, an anionic layer of dextran sulphate, to change the direction and magnitude of the flow. Both of these coatings are shown to reproduce electroosmotic flow for a wide range of pH. In a hydrodynamically driven capillary zone electrophoresis, electroosmotic flow needs to be suppressed. Kaniansky and coworkers worked on eight different electroosmotic flow suppressors and have shown influences of these on electrophoretic separation efficiencies at different pH values (Kaniansky et al., 1997). Ramsey and coworkers have developed a microfluidic device to detect Escherichia coli (E. coli) using pure electrophoretic transport. They were able to suppress electroosmosis, using poldimethylacrylamide (McClain et al., 2001). (Barker et al., 2000) have used polyelectrolyte multilayers to alter the electroosmotic flow direction in polystyrene and acrylic microfluidic devices. They were able to achieve complex flow patterning, and flow in the opposite directions of the same channel.

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