Theoretical and Experimental Studies on Electroosmotic Membrane Nanopumps

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Theoretical and Experimental Studies on
Electroosmotic Membrane Nanopumps

• Zuli XU and Ping SHENG
• Department of Physics,
• Hong Kong University of Science and Technology,
  Clear Water Bay, Hong Kong, China

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Collaborators

• Dr. Jianying MIAO (NAMI, HKUST)
• Prof. Ning WANG (Physics, HKUST)
• Prof. Zhiyu YANG (Physics, HKUST)
• Prof. Ping SHENG (Physics, HKUST)

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I. Motivations

• Potential applications of the EO micro/nanopumps
• Liquid drug delivery with fully controllable,
  large dynamic range of pumping rates
• Microfluidics and nanomachine applications
• Pumps for inks of electronic paper displays
• Micro-electronic cooling

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II. Theoretical Studies on the Electroosmotic
Micro/nanopump

• Schematic structure and electrical potential
  distribution of EO pump

Charged surface creates a double layer of ions
and a potential difference.
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Mechanism of Electroosmotic Flow
Electroosmotic flow is the motion produced by the
action of an applied electric field on a fluid
with a net charge.
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Electric Potential Distribution
Assume the equilibrium Boltzmann distribution
equation to be applicable,

• Poisson equation

Dimensionless Poisson-Boltzmann equation
Dimensionless
Debye length
Boundary conditions

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Velocity Profile
Poisson-Boltzmann Equation
Navier-Stokes Equation
Boundary conditions
Boundary conditions
No Slip B.C.
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Efficiency of Nanopump Coupled to an External
Load
Conversion efficiency of EO pumps when coupled
to an external load is defined as the ratio of
mechanical hydraulic power output, as seen by the
external load, to electrical power input, where V
is the applied voltage on the channels
Volume Flow Rate
Total Electric Current

• The current density
• where is the mobility of the
  respective positive and negative ions
• (Here D is the binary diffusion coefficient of
  the ion in the solvent).

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Calculation results of maximum efficiency
Maximum external work efficiency VS the ratio of
the Debye length to the pore radius
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II. Experimental Studies on AAO membranes

• Schematic Experimental Setup

Nanopump made by anodic porous alumina

1. SEM and AFM (inset) images showing the uniform
   nanochannels in the AAO membrane prepared by
   anodization.
2. TEM image of AAO channels coated with silica.

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Experimental results of factors influencing the
flow rate of DI water

• Changing pore sizes of AAO films

Micropumps Based on the Enhanced Electroosmotic
Effect of Aluminum Oxide Membranes, J.Y. Miao,
Z.L. Xu, X.Y. Zhang, N. Wang, Z.Y. Yang, and P.
Sheng, Adv. Mater. 19, 42344237, (2007)
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Experimental results of factors influencing the
flow rate of DI water

• Surface treatment conditions of AAO films

Micropumps Based on the Enhanced Electroosmotic
Effect of Aluminum Oxide Membranes, J.Y. Miao,
Z.L. Xu, X.Y. Zhang, N. Wang, Z.Y. Yang, and P.
Sheng, Adv. Mater. 19, 42344237, (2007)
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III. Conclusion

• Use anodic aluminum oxide (AAO) films as the
  template to fabricate the EO micro/nanopumps
• Enhance the zeta potential of the surface to
  improve the performance of EO pump, such as
  coating of different materials on the inner walls
  of AAO channels(AAO with silica coating to the
  channels, the pump can produce a maximum pressure
  of 1 atm and a maximum flow rate of 86,000
  ?L/mincm2 under an applied field of 0.94 V/ ?m).
• In order to obtain the optimal EO pumps, the
  radius of the micro/nanochannel should be on the
  order of 2.5 times the Debye length.

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Thank you!
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Efficiency of EO Pump
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Micropumps

• Mechanical
• relatively large size and large flow rates
• Non-mechanical
• electrokinetic
• electrohydrodynamic
• magnetohydrodynamic
• electro-wetting
• electrochemical pumps
• generally low maximum flow rates and high
  operating voltages

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Our Effort

• Use anodic aluminum oxide (AAO) films as the
  template
• Electrodes for applying voltage across the film
• Coating of different materials on the inner walls
  of AAO channels to activate the pump
• Working medium, such as DI water, dilute acid,
  dilute base and neutral salt solution, to supply
  the working fluid for the pumps

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Advantages of AAO Membrane as Micro/Nanopump

• Naturally formed straight pores
• Relatively low operating voltage
• Large dynamic range of flow rates
• Very thin
• Adaptable to small area or large area applications

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Survey of Some Efforts on Electroosmotic Pumps

• Electroosmotic (EO) pump fabricated with
  soda-lime glass substrate
• maximum pressure 0.33 atm
• maximum flow rate 15 µL/min at 1 kV
• Ref A Planar Electroosmotic Micropump, C.-H.
  Chen and J. G. Santiago, Journal of
  Microelectromechanical Systems 11, 672-683
  (2002)
• Quartzplate dry-etched with a neutral loop
  discharge of CF4 and C3F8
• 800 Pa static pressure
• flow rate 415 nL/min at 10V
• Ref Low-voltage Electroosmosis Pump for
  Stand-alone Microfluidics Devices, Y. Takamura,
  H. Onoda, H. Inokuchi, S. Adachi, A. Oki, Y.
  Horiike, Electrophoresis 24, 185-192 (2003)
• A silica matrix with 100 µm pores
• maximum flow rate 2.9 µL/min
• maximum pressure 3 atm for DI water at 6kV
• Ref A New Electro-osmotic Pump Based on Silica
  Monoliths, P. Wang, Z. Chen and H.-C. Chang,
  Sensors and Actuators B113, 500-509 (2006)
• Fused-silica capillaries with non-porous silica
  particles
• pressure 20 atm
• flow rate 3.6 µL/min at 2 kV
• Ref Fabrication and Characterization of
  Electroosmotic Micropump, S. Zeng, C.-H. Chan, J.
  C. Mikkelsen Jr. and J. G. Santiago, Sensors and
  Actuators B79, 107-114 (2001)
• Nanopumps made of carbon nanotube membrane in
  porous alumina
• flux 237.9 nmol/cm2 in electrolyte solutions
• Ref Electroosmotic Flow in Template-Prepared
  Carbon Nanotube Membranes, S. A. Miller, V. Y.
  Young and C. R. martin, J. Am. Chem. Soc. 123,
  12335-12342 (2001)

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Factors influencing the flow rate of DI water

• Heat-treatment temperature of AAO films

Pore diameter 208 nm Thickness 83 um
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Factors influencing the flow rate of DI water

• Flow direction

Sample Thickness (um) Pore size (nm)
Ratio of pore diameter on one end to that on the
other end A 20 265 166
1.7 B 15 265 91
3.2
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Factors influencing the flow rate
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Influence of Acidity
Pore diameter 50nm Thickness 30um Electric
field 20V/30um 0.67V/um
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Factors influencing the flow rate of DI water

• Performance of annealed AAO film micro/nanopumps

As- prepared (A) As- prepared (B) As- prepared (C) H2O2 treated H2SO4/ Na2Cr2O7 treated Silica Coating
Flow Rate (mL/cm2.min) 0.08 - 0.23 0.25 - 0.65 0.61 - 2.25 1.3 - 13 9.6 - 22 4.4 - 86
Pore Fraction 20 27 30 30 30 35
Diameter (nm) 50 100 200 200 200 280
Thickness (µm) 30 60 25 25 25 74
Center to Center Distance of Pores (nm) 125 190 330 330 330 438
Electric Field (V/µm) 0.17 - 0.67 0.17 - 0.75 0.20 - 1.2 0.4 - 1.2 0.4 - 1.4 0.14 - 0.94
Best efficiency 1
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Our Efforts

• In order to obtain the large flow rate, we change
  the dimensionless Debye length (diameter of
  channels, etc) and the zeta potential.
• Diameter of the AAO nanochannels ranges from
  60300nm (but uniform-sized), with thickness of
  30100?m.
• Chemical treatments for the AAO channel
  surfaces, such as coating silica on the inner
  walls, to get the optimal zeta potential.

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Present work
Main Process Flow of Silicon Wafer
Etching Substrate Silicon wafer, 4, 400um
thickness with 3um Oxide (Double Sides)

• Photolithography
• Oxide Etching
• Photoresist Stripping
• Silicon etching (ICP DRIE )
• Oxidation

Si
Silica
Schematic process flow
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SEM image of artificial microchannels on silicon
wafer