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:

1.     Wouden, E. et al. Field-effect control of electro-osmotic flow in microfluidic networks. Colloids and Surfaces A: Physicochemical and Engineering Aspects267, 110-116 (2005).

2.     Wu, W.-I., Selvaganapathy, P.R. & Ching, C.Y. Transport of particles and microorganisms in microfluidic channels using rectified ac electro-osmotic flow. Biomicrofluidics5, 013407 (2011).

3.     Wu, W.-I., Sask, K.N., Brash, J.L. & Selvaganapathy, P.R. Polyurethane-based microfluidic devices for blood contacting applications. Lab on a chip12, 960-70 (2012).

 

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