Workshop proceeding - final.pdf - Faculty of Information and ...

Figure 8. Particle behaviours at frequencies from 100 Hz to 1 MHz. (A-D): positive DEP observed; (E
& F): close to crossover frequency. (G): typical negative DEP behaviour. (H & I): DEP behaviour of
particles at 500 kHz and 1 MHz is similar to that at 150 kHz.
5.3 Particle Separation
The ability to use DEP to sort particles was investigated by applying electric fields with
frequencies of 100 Hz, 1 kHz, 100 kHz and 150 kHz and examining the system for spatial separation
of the particles. The separation progress in the stationary liquid is shown in Figures 9 and 10.
At 100 Hz (Figure 9-A), strong positive DEP was observed. Large numbers of polystyrene
microparticles and MWCNTs were in constant motion in the region near the electrode tips. This was
due to the existence of large levitating and trapping forces described in Section 3. When the frequency
of applied field was increased to 1 kHz (Figure 9-B), microparticles in motion could no longer be
observed. As this frequency increased to values closer to the crossover frequency (approximately 30
kHz), the DEP force became weaker and the number of assembled microparticles decreased. Figure 9-
C illustrates initial separation behaviour of polystyrene microparticles and MWCNTs at 100 kHz, as
all polystyrene microparticles were repelled from strong electric field region, while MWCNTs stayed
on the edge of the electrodes. At 150 kHz (Figure 10-A), the separation of MWCNTs and polystyrene
microparticles was complete. Since the magnitude of DEP forces affecting MWCNTs and polystyrene
microparticles is not changing for frequencies larger than 100 kHz, applied signals in the range of 100
kHz to 1 MHz resulted in similar DEP behaviour (Figure 9-D illustrates the particle behaviour at 1
MHz).
After the polystyrene microparticles were removed and the liquid was evaporated, the substrate
was characterised by a scanning electron microscope (SEM) to observe the trapping effect of
MWCNTs; and as can be in Figure 10-B, MWCNTs were trapped on the tip and edges of electrodes.
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