Electroosmotic (EO) pumps (a.k.a. electrokinetic pumps) have no moving parts and are capable of generating high flow rate per device volume. They use electroosmotic pumping to achieve both significant flow rates and pressures, and a fairly wide range of working electrolytes may be used (including deionized water, buffered aqueous electrolytes, methanol, acetone, and acetonitrile). These devices have significant pressure capacity in a compact structure. We have achieved flow rates in excess of 50 cc/min have been achieved with pumping with disk-like structures less than a cubic centimeter in volume and with 100 V applied potentials. Pressures in excess of 200 atm are also possible using micro- and millimeter-scale packed-column pumps with 10’s of kilovolts applied potentials (the latter generating order 10 nl/min). They offer some advantages over other miniature pumps for microchannel cooling applications and integrated bio-analytical systems. Below is a brief history and review of EO pump research :
Figure 1. History of EO pumps including patent ideas and research in porous glass pumps and pumps microfabricated in glass and silicon. Proposed applications include pumping for a liquid chromatography separation (Pretorius et al., 1974), controlled drug delivery systems (Theeuwes et al., 1974), miniaturized actuation devices and high-pressure valves (Paul et al., 1998), and electronics cooling (Jiang et al., 2002).
Figure 2. Laser and Santiago (2004) presented a review of miniature pumps including comparisons of EO pumps to other miniature pump technologies. Self-pumping frequency is here defined as maximum flow rate divided by micropump device volume. The range of maximum pressure is also shown in the legend (indicated by size of data symbol).
EO pumping in porous glass structures
We treat EO flow in porous media as flow through a large number of idealized tortuous channels in parallel, as depicted in Fig. 3. Tortuous microchannels (i.e., pores) with circular cross sections are assumed to have equal pore radii, a, and an equal value of zeta potential, z. This model is leveraged to derive a set of analytical solutions for the maximum flow rate, maximum pressure, and electric current.
Figure 3. Schematic of the porous model with idealized cylindrical pores of uniform diameter. Flow is modeled within each pore (as shown on the right) and then flow rate and current are integrated over all pores in the system. The structure can be characterized by its total volume, ", the void volume, "e, length, L, and tortuous characteristic length of the pores, Le. The ratios "/"e and (Le / L) are defined as the porosity and tortuosity, respectively.
Three critical length scales
EO pump devices require the interaction of physical phenomena across a wide range of length scales. In particular, there are three critical length scales required for operation. Change of any of these three by, say, an order of magnitude and system performance is completely compromised. These length scales are
1. The electric double layer length scale (i.e., the Debye length):
Figure 4. Structure of the electric double layer near a solid/liquid interface. When external electric field is applied, bulk motion of an electrolyte caused by Coulombic forces acting on ions in the electric double layer. The typical length scale for the Debye length is on the order of 1 nm, depending on the ionic contents in the liquids.
2. The pore diameter, which determines maximum achievable pressure per applied volt:
Figure 5. A scanning electron microscope image of the porous glass structure. This structure provides the high wetted-surface-to-volume ratio required to generate high pressures.
3. The macroscopic flow area which determines maximum flow rate capacity:
Figure 6. Image of the porous glass structure. The flow rate per applied electric field is linearly proportional to the macroscopic flow area.
EO pump design and operation
We have built porous-structure EO pumps by packing and sintering 3-6 um silica and borosilicate glass particles. We have also created pumps with commercially available porous glass materials. One commercially available filter structure is manufactured by ROBU Glasfilter-Geraete (GmbH, Germany) and shown in the Fig. 5 along with a scanning electron microscope image of the porous glass structure in Fig. 6.
These glass filters, or “frits”, are typically 40 mm in diameter, 1-5 mm thick, and with a mean pore diameter of 1-2 um. We have used this particular frit structure to realize sealed cooling systems of microelectronics with 140 W heat dissipation (see Jiang et al., 2002 & Yao et al., 2003). We have validated the performance of these pumps using detailed measurements of pressure, flow rate, current, and thermodynamic efficiency. Yao et al. 2003 describes results for detailed parametric study including results from six geometrically different pumping devices, applied voltage variations, and 13 electrolyte ion densities.
Figure 7. Schematic of the EO pump device. A gas-permeable Teflon (PTFE) membrane directs most of the downstream hydrogen gas to bypass the fluidic circuit and flow into the catalytic chamber on the high-pressure side of the device. A platinum catalytic surface within the pump structure recombines electrolytic hydrogen and oxygen gas into water, which is then reclaimed by the system.
8. Image of the pump components showing
flow connections and device power leads. Scanning electron microscope image of a
porous structure is shown on the right.
Figure 9. Normalized flow rate and pressure performance of the porous glass EO pump across various parameters (pump length, pore size, and applied potential). In all cases, working fluid is 1 mM (sodium ion concentration) borate buffer (pH 9.2, s¥ = 77 uS/cm) and applied voltage ranged from 5 to 100 V (yielding respective effective voltages ranging from 0.5 to 86 V). These data are all linear with respect to the value of the abscissa, as predicted by our analytical model. The flow rate is linear with the applied electric field and the pressure is linear with the applied voltage.
Electronics cooling application
In collaboration with Stanford’s Kenny and Goodson groups, we have applied our EO pump technology to the cooling of microelectronics. A schematic of this principle is shown below.
Figure 10. Schematic of EO-pumped, liquid cooling loop for microelectronics.
Below is an example application of our technology in which we replaced the fan and heat sink on a Sony Vaio with an EO pump that circulates water over a micromachined heat exchanger and rejects the heat from the panel behind the screen:
Figure 11. Working laptop with integrated EO pump cooling system consisting of an EO pump powered using the device fan leads (now removed) and a DC-to-DC converter, a microchannel heat exchanger (this fabricated by the frm Kenny and Goodson groups), and a heat rejector integrated into the panel behind the LCD display.
To read more about this research, see references and websites below.
Zeng, S., Chen, C., Mikkelsen, J.C., and Santiago, J.G., 2001, “Fabrication and characterization of electroosmotic micropumps”, Sensors and Actuators B, 79 (#2-3), 107-114.
Zeng, S., Chen, C., Santiago, J.G., Chen, J., Zare, R.N., Tripp, J.A., Svec, F., and Frehet. J., 2002, “Electroosmotic Flow Pumps with Polymer Frits”, Sensors and Actuators B, 82(#2-3), 209-212.
Jiang, L., Mikkelsen J. C., Koo, J.-M., Huber, D., Yao, S., Zhang, L., Zhou, P., Maveety J. G., Prasher R., Santiago J. G., Kenny T. W., and Goodson K. E., 2002, “Closed-Loop Electroosmotic Microchannel Cooling System for VLSI Circuits”, IEEE Trans. Comp., Packag., Manufact. Technol. 25(#3), 347-355
Yao, S., and Santiago, J. G., 2003, “Porous Glass Electroosmotic Pumps: Theory”, Journal of Colloid Interface Science, 268, 133-142.
Yao, S., Hertzog, D. E., Zeng, S., Mikkelsen, J. C., and Santiago, J. G., 2003, “Porous Glass Electroosmotic Pumps: Design and Experiments”, Journal of Colloid Interface Science, 268, 143-153.
Laser, D. and Santiago J. G., 2004, “A Review of Micropumps”, J. Micromech. Microeng. 14, R35-64.
Other electroosmotic/electrokinetic pumping web sites:
“It flows purling, widely flowing, floating foampool, flower unfurling.”
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