(8i) Pressure Generation Using Rectified AC Electroosmotic Flow with Field Effect Flow Control

PRESSURE GENERATION USING
RECTIFIED AC ELECTROOSMOTIC FLOW WITH FIELD EFFECT FLOW CONTROL

Wen-I
Wu
and P. Ravi Selvaganapathy

Department
of Mechanical Engineering, McMaster University, CANADA

Rectified AC
electroosmotic (EO) pumps using field-effect flow control (FEFC) have several
advantages over DC EO pumps such as electrolysis elimination and low driving
voltages[1],
however it has not been fully characterized. Prior literature[1][2]
focused
on flow generation and calculated pressure heads based on the maximum flow
rate(). Here, we
report the first experimental demonstration of pressure generation using FEFC
mechanism. EO flow is extremely sensitive to the back pressure, thus pressure
conversion using the maximum flow rate could introduce certain errors and
overestimate the performance of AC EO pumps.

This paper first
details the development of the rectified AC EO microfluidic pump (Fig.1) with
the ability to alter the cross-section area using a clamping mechanism, and
then employs three methods to directly measure pressure head. The rectified
flow is obtained by synchronous zeta-potential modulation with the driving
potential in the microchannel. This pump is composed of 10 microchannels
(L10mmW150µm) with an initial height of 20µm and is fabricated using the
hydrophilic polyurethane (PU)-based microfabrication[3] to
facilitate channel priming. In addition, elastic PU allows modulation of the
channel cross section by external clamping which facilitates the
characterization of the effect of channel cross section on the pressure
generation capability of the micropump (Fig.2). Using FEFC, the flow rate and
pressure output from this pump can be controlled through the driving electric
field E_d (Fig.3a), applied frequency f (Fig.3b), gate
potential V_g (Fig.4a), and phase lag between the driving and
gate potentials Φ (Fig.4b). As seen in Fig.3, the rectified AC EO
flow rate also depends on the cross-section area of the channel A which
can be estimated by  (µ_EO:
EO mobility)

To characterize
the pressure head, direct measurements including hydrostatic height, Boyle's
law and pressure gauge were employed to eliminate the measurement errors
(<10% variation). Working solutions including DI water, 0.1mM and 10mM PBS
were tested. The current threshold for bubble generation was characterized so
that the rectified AC EO pump is operated bubble-free. The comparison of
pressure head generation is plotted in Fig.5. The maximum pressure head
decreases with the applied frequency and the ionic strength of working solution
as expected due to the slow EOF response time and thinner electrical double
layer respectively. When frequency is low as 1Hz, the results from experimental
measurement and calculation are similar, however the discrepancy increases with
the applied frequency which indicates the commonly-used Q_max conversion could only be valid for
DC or low frequency AC EOF, but not applicable for higher frequency. A maximum
pressure output of 2.7kPa can be obtained from this device with 1Hz
square-waveform signals, 1000V/cm driving electric field, 1500V gate
potentials, and DI water as working solution without bubble generation over
long periods of time (4hrs). Despite the relatively high gate potential which
can be reduced substantially by thinner dielectric layer (25µm PU here), this
rectified AC EO pump can be operated free-of-bubble with the capability of pressure
generation, thus can be applicable for water removal in fuel cells and
microfluidic control in drug delivery applications.

REFERENCES:

Figure 1: (a) Illustration of microfluidic pump; (b)
appearance of microfluidic pump with clamp

Figure 2.Schematics of (a) top
view; (b) side view before clamping and (c) after clamping

Figure 3.Effect
of (a) the driving potential and (b) the applied frequency on the rectified AC EOF for various
cross-section areas

Figure
4.Effective of (a) gate potential and (b) phase lag on rectified AC EOF under
the minimum cross-section area (A=1841µm²)

Figure
5.Pressure heads estimated by Q_max conversion and
direct measurements at different applied frequencies when E_d=
1000V/cm and V_g= 1500V