St = L2/ I7 (5-5)
As the Reynolds number is less than the order of unity in a typical EOF; the second term in
the momentum equation can be dropped off. However, the Strouhal number, as the ratio of time
scale of EOF to the period of the applied electric field can be large, making the temporal term in
the momentum equation dominant.
5.3.2 Pulsed Recirculation
The application of a constant pressure and such a pulsed electric field results in a time
periodic flow in the ridged channel. Each period consists of two phases: recirculation phase
when the electric field is on, and feeding phase when the electric field is off. In the recirculation
phase, recirculation occurs in the flow and the fluids in the ridged channel are homogenized due
to the actuation; the mixer is active. While in the second phase, the flow restores to laminar and
thus the mixer is inactive. During the feeding phase, the mixed fluids are flushed out of the
ridged channel, and unmixed fluids are fed into the mixer.
The mixing is related to the manner that electric field is applied, namely the magnitude of
electric field, the oscillation frequency, the wave form and the duty cycle. The optimization of
mixer is not performed in this work, and only guidelines are discussed here. In general the
following operation conditions are desired in order to obtain an optimal mixing.
Strong recirculation intensity. The stronger the recirculation, the more effective the
mixer is. Based on the numerical study of flow recirculation in the ridged channel, the intensity
of recirculation depends on the ratio of the external forces imposed on the fluid. The amplitude
of the electric signal should be adjusted such that in the recirculation phase, the resulting
dimensionless parameter, K, is in the range where significant flow recirculation is obtained.
Sufficient recirculation time. The length of the recirculation phase ought to be long
enough to ensure a fully development of the flow recirculation in the ridged channel and a