Interdisciplinary Applied Mathematics

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(Jones et al., 2001) utilized dielectrophoresis to create water droplet volumes ranging from microliters down to nanoliters. They have shown that dielectrophoresis provides a means for manipulation of small water volumes, against overwhelmingly strong surface tension and capillary effects. Their method enables rapid motion of water in capillaries with smooth substrates, and creation of multiple nanoliter droplets in less than 0.1 s. A great potential of this method is that droplet motion across a substrate for diagnostics, mixing, separation, and dispensing purposes is possible. However, the foremost advantage of this new method is “division of an initial liquid inventory into discrete droplets before processing, which avoids sample cross contamination” (Jones et al., 2001). In addition to the DEP and the opposing capillary and surface tension effects (see Chapter 8), wetting, transient fluid motion, Joule heating (see Section 7.4.6), and RF discharge are also important. More research is required for further understanding and accurate modeling of this phenomenon (Jones, 2001). In other work, Velev and coworkers described dielectrophoretic manipulation of freely suspended droplets in a liquid-liquid microfluidic system (Velev et al., 2003). Unlike the previous work, this new approach eliminates droplet contact with the electrodes by suspending the droplets in an immiscible fluid. They have reported manipulation and mixing of micro to nanoliter volume droplets. These microfluidic devices can be utilized in a range of applications, including the synthesis of new materials and development of biological microassays (Velev et al., 2003).

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FIGURE 7.25. Particle fluorescence image of filamentary (upper) and trapping (lower) dielectrophoresis. Regions of high particle concentration emit intense fluorescence. The flow is from top to bottom produced by an applied field of 25 V/mm and 100 V/mm, for the upper and lower figures, respectively. The circular posts have diameter 33 ^m with center separation of 63 ^m. (Courtesy of E. Cummings.)

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