Biomedical Engineering Reference
In-Depth Information
Figure 10 . AC electrokinetically driven particle motion: ( a ) experimentally measured
particle velocity field using micro-PIV, and ( b ) numerically simulated particle velocity
arising from electrothermal fluid motion and viscous drag. This suggests that for this re-
gime ETF is dominant compared to DEP.
typically causes electrothermal effects. The electrodes are arranged in interdigi-
tated pairs so that in Figure 11 the first and third electrodes are always at the
same potential as each other. The second and fourth electrodes are also at the
same potential as each other but can be at potential different from the first and
third electrodes. An alternating electric potential is applied to the interdigitated
electrodes to create an electromagnetic field with steep spatial gradients. Particle
motion through the resulting electric field gradients causes polarization of the
microspheres, resulting in a DEP body force that repels particle motion into in-
creasing field gradients.
Six sets of experiments were performed, each using a different voltage. All
experiments used the same flow rate of 0.42 l/hr (equivalent to a Reynolds
number of 3.3 + 10 -4 ) and the same AC frequency of 580 kHz. The voltages
were chosen between 0 and 4 V. Charge-neutral fluorescent polystyrene parti-
cles measuring 0.69 m in diameter (Duke Scientific) were suspended in the
flow. Sets of images 800 images each were acquired using a Photometrics
CoolSNAP HQ interline monochrome CCD camera from Roper Scientific. This
camera is capable of 65% quantum efficiency around the 610-nm wavelength,
which is the emission wavelength of the red fluorescing microspheres. Images
were captured at a speed of 20 frames per second. The microscope used in these
experiments was an epifluorescent Nikon E600 with a Nikon "CFI W FLUOR
60X" water-immersion objective lens having a numerical aperture of 1.00. Epif-
luorescent imaging was used because the silicon base of the device is highly
reflective, resulting in strong background noise. The broad-spectrum mercury
vapor light source is bandpass filtered to admit wavelengths of approximately
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