Biomedical Engineering Reference
In-Depth Information
is being studied, the technique is the same. The small particles are observed with
a microscope. The particles are typically coated with a fluorescing dye to enable
epifluorescent imaging. Images are captured with a precise time delay from one
image to the next. Consecutive pairs of images are divided into many small in-
terrogation regions. The corresponding interrogation regions from each of the
two original images are cross-correlated to determine the most likely relative
displacement of particles in the interrogation regions in the form of a cross-
correlation peak. Repeating this procedure thousands of times produces the spa-
tially resolved measurements of fluid or particle motion seen in the following
sections.
4.2. Electrothermal Effect
Micro-PIV experiments using polystyrene spheres in an optically accessible
flow cell with wedge-shaped electrodes have been conducted. The trajectories of
1- m diameter polystyrene particles suspended in sugar solution were measured
in a device consisting of two brass electrodes sandwiched between two glass
wafers. An AC potential of 10 V rms at 10 kHz was applied to the electrodes. The
particle-velocity field is measured quantitatively using NPIV following Meinhart
et al. (11), and is shown in Figure 10a.
The experimental results compare well to numerical solutions of electro-
thermally driven flow: fluid motion is simulated by solving the Stokes equation,
subject to an electrothermal force (Eq. [13]). The velocity of suspended 1- m
particles relative to the fluid medium can be estimated by balancing the two
dominant particle forces: Stokes drag force and DEP force. The numerically
simulated particle velocity field is shown in Figure 10b. For these parameters,
according to model results, the DEP was negligible in comparison with motion
generated through electrothermal flow. The results are described in detail by
Meinhart et al. (12). The agreement between simulations and experiments may
indicate that electrothermal forces are important in the microfluidic devices
tested. However, in these numerical simulations, the effect of AC electroosmosis
is not modeled.
4.3. Dielectrophoretic Effect
The second set of experiments were designed to isolate the effects of dielec-
trophoresis from electrothermal motion. A channel measuring 350 m wide by
12 m deep is shown in Figure 11. The bright regions in the image are platinum
electrodes, while the dark regions are areas without platinum or electrode gaps.
The entire imaged region is covered by a thin layer of silicon dioxide, which
insulates the electrodes from the fluid medium to suppress the Joule heating that
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