Interaction of Water with Nanoporous Substrates

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Interaction of Water with Nanoporous Substrates

• Elizabeth Clark

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Acknowledgments

• Dr. John Barnard Advisor
• Mr. Albert Stewart SEM Images
• Lawrence Weinstein and Sean Thaler Sputter
  coating of substrates

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Motivation

• In recent years, scientists have been
  investigating surface phenomenon, including those
  surfaces which produce high contact angles
  
• An understanding of wetting behavior is
  fundamental to water flow, water treatment, and
  water contamination with new nano-materials.
  
• Previous studies have prepared ultrahydrophobic
  surfaces using elaborate techniques such as argon
  plasma etching2.
  
• However, in this experiment, commercially
  available alumite filters, sputter coated with
  different gold thicknesses, were used to observe
  high contact angles.

In nature, there are some plants such as the
lotus (above) whose leaves exhibit high contact
angles due to surface roughness on a micrometer
level1.
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Interaction of Water with Nanoporous Substrates

• The interaction of water with nanoporous alumite
  filters was studied by observing the contact
  angle on coated alumite filters.
  
• Several equations are used to explain the contact
  angle
  
• Youngs Equation
• Wenzels Equation
• The Cassie-Baxter Equation

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Youngs Equation3

• Youngs Equation
• Equilibrium contact angle of a smooth,
  homogeneous surface

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Wenzels Equation4

• Wenzels equation gives the contact angle on a
  rough, homogeneous surface
  
• cos ?rough r cos ?
• r-roughness factor the actual surface
  area/geometric surface area
  
• ?-contact angle on smooth material

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The Cassie-Baxter Equation4

• The Cassie-Baxter equation gives the contact
  angle on a porous, homogeneous material
  
• cos ?porous f1cos? f2
• f1-fraction of fluid area in contact with the
  material
  
• f2-fraction of fluid area in contact with the air
  in pores

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Drop Shape Analysis System (DSA 100)
Needle and Dispensing System
Camera
Image
Sample Stage
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Experimental Procedure

• Advancing contact angle measurements were made
  by
  
• Producing a drop 3.5µl in size
• Positioning the needle in the center of the drop,
  just above the substrate
  
• Adding 0.2µl of water to the drop, and using the
  DSA 100 to measure the contact angle as it
  advanced
  
• Recording the largest contact angle measurement
• Evaporating Drops were observed by
• Dispensing a 0.4µl drop on the substrate
• Using the DSA 100 to record a contact angle and
  drop diameter measurement and to take an image of
  the drop regularly until the drop evaporated

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Substrates

• Two substrates were used
• Standard Silicon wafers with an oxide layer on
  the surface
  
• Whatman Anodisc filters
• These porous alumina discs come in pore sizes of
  20nm, 100nm, and 200nm.
  
• The anodiscs and silicon wafers were
  sputter-coated with gold to achieve three
  different thicknesses 10nm, 50nm, and 100nm

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Sputter-Coating

• In the sputtering process, the gold atoms do not
  all follow a normal path to the substrate. In
  fact, the gold particles follow a Gaussian
  distribution as they deposit on the surface.
• The range in deposition angles contributes to a
  slight build-up of gold on the substrate walls,
  and so the pore-size can be controlled.

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20nm Alumite 100nm Alumite 200nm
Alumite
SEM Images of Gold-Coated Alumite Substrates
10nm Gold Coat
50nm Gold Coat
100nm Gold Coat
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Advancing Contact Angles Results
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Advancing Contact Angles on Uncoated and Gold
Coated Substrates
Porous Alumite (20 nm)
Porous Alumite (200 nm)
Flat Si wafer
No Gold
10 nm Gold
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Evaporation Constant Contact Angle (Flat
Substrate)
Water on a silicon substrate with a 10nm thick
gold coating.
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Evaporation Constant Drop Diameter (Porous
Substrate)
Water on an alumite disc with a 10nm thick gold
coating.
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Summary of Results

• High contact angles were created by gold-coating
  the nanoporous substrates
  
• However, the receding contact angles as seen in
  the evaporation of the drops, tended to zero for
  the nanoporous substrates.
  
• The low receding contact angle may be due to the
  pores, which can pin the contact line
  
• Because of the low receding contact, these
  substrates are not water repellent they do not
  slide off the surface.

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Implications in Nanotechnology and the Environment

• Catalysts
• Different catalytic effects based on size, shape,
  composition, and surface properties6
  
• Water Filters
• Ceramic filters with nanosized pores can filter
  out bacteria and viruses7
  
• Self-Cleaning Surfaces
• Can use a surface with a high (many waterproof
  surfaces) or low (TiO2) contact angle
  
• Biomedical Applications
• Scaffolds for organ repair
• Release of medicine through time

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Future Projects

• Repeat the experiment with different liquids
• Repeat the experiment with nanoparticles in the
  liquid or on the substrate
  
• Observe how water interacts with nanoparticles

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Works Cited

• Blossey, R. Nature Materials 2003, 2, 304.
• Chen, W. Fadeev, A.Y. Hsieh, M. C. Öner, D.
  Youngblood, J. P. McCarthy, T. J. Langmuir 1999.
  15, 3395.
  
• Tadmor, R. Langmuir 2004, 20, 7659.
• Youngblood, J. P. McCarthy, T. J. Macromolecules
  1999, 32, 6800.
  
• Dr. Barnard Paper
• Larsen, S. C. In Nanotechnology and the
  Environment Applications and Implications Karn,
  B. Masciangioli, T, Zhang, W. Colvin, V.
  Alivisatos, P., Ed., American Chemical Society
  Washington, D. C., 2005 p 268.
• Schulenburg, M. Nanotechnology Innovation for
  Tomorrows World, European Commission, Research
  DG Berlin, 2004 p 38.