hydraulic diameter is 56 micron. A time varying electric field is imposed by applying a square
wave electric potential at the outlet of the mixer and keeping the two inlets grounded.
The mixing process of two segregated streams is simulated. The incoming streams,
deionized water and a 0.1 mM Rhodamine-B solution are pumped by an average pressure drop of
375 Pa/cm. The diffusion coefficient of Rhodamine-B is set as 3.6x10-10 m2/s. An electric field
of mean strength of 272 V/cm is applied. Based on the definition of K in Equation 4-32 the
corresponding parameter K is 9.1, roughly in the range where significant flow recirculation
would be obtained. The electric field is alternating at the frequency of 1 Hz, and the duty cycle
is 50%. Therefore in each cycle the duration for recirculation and feeding phases are 0.5 second
each. The fluid density is set as 997 kg/m3, and the fluid kinetic viscosity is 1x10-6 m2/s in the
simulation. With the simulation configuration, the criteria for optimal mixing in Section 5.2 are
all satisfied.
The initial state of simulation is set to be a steady flow where zero electric field is applied.
At zero second, the pulsed signal is applied. The fluid flow in the PRM is calculated every 0.1
second by the CFD software, and the simulation stops at 6 seconds, the end of the 6th period.
Simulation shows that flow recirculation occurs in the PRM at a frequency of 1 Hz, and the
unsteady flow quickly settles at an oscillating pattern as the transient effect decays and becomes
negligible.
Figure 5-12 shows the typical evolution of the distribution of Rhodamine-B at a cross
section over one period. The cross section is located at -1 mm after the merging of two streams,
between the 4th and 5th ridge structures (section II in Figure 5-11). Contour lines of Rhodamine-
B concentration are calculated and plotted at the cross section views for each frame. Also shown
in Figure 5-121is the contour lines in a flow at the cross section of a regular channel (without