Biomedical Engineering Reference
In-Depth Information
FIGURE 7.19
Dimensionless time-dependent velocity profile of an electrokinetic flow in an AC electric field (b
¼
5).
behavior. Thus, an AC electric field can also be applied in flow direction for creating instability and
improving mixing.
7.5.3 Instability caused by variation of zeta potentials
Pulsating electrokinetic flow can also be generated by keeping the electric field constant while varying
the zeta potential at the channel wall ( Fig. 7.20 ). Zeta potential on channel walls can be controlled by
shielding electrodes, in the same way as field effect transistors work [23] . Controlling these shielding
electrodes lead to spatiotemporal changes of zeta potential in mixing channels, and thus instability in
the mean flow.
The shielding electrode and the channel wall form a capacitor, where the dielectric medium is the
device material. Considering a shielding electrode parallel to the channel wall under steady-state
condition, the charge density stored in the capacitor is:
3 r 3 0 ð
V c
V i Þ
r el ; 0 ¼
(7.53)
d
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