The Hydrophobic Characteristics of Aerosol Gel

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The Hydrophobic Characteristics of Aerosol Gel
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Research Summer 06

• Goals
• Understand the hydrophobic behavior of the
  aerosol gel
• Determine variables that affect hydrophobicity
• Ultimately learn how to manipulate surfaces and
  make them hydrophobic
• Applications
• Low friction surfaces for submarines, deicing
  airplane wings, water repulsion for rain coats

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Aerosol gel characteristics

• Fractal Aggregate
• Density approximately 2.0 mg/cc
• High surface area
• Electrically conductive
• Hydrophobic

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Definitions

• A hydrophobic surface is a surface that the
  contact angle of a water droplet on it gt 90
• The contact angle is a measure of the
  hydrophobicity of the surface
• Contact length is the diameter of the base of the
  drop that contacts the surface (see annotation)

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Theory of hydrophobicity

• Ideas of surface tension/surface energy between
  phases yield the Young relation, assuming a
  smooth and chemically homogeneous surface
• cos ?E (?SV?SL)/?LV

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Refining Theory(a.k.a. Taking reality into
account)

• Wenzels relation was developed as an attempt to
  understand the affect of rough surfaces on
  hydrophobicity

cos ? r cos ?E
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More refining
cos ? f1cos?1f2cos?2

• The Cassie-Baxter relation was developed to deal
  with heterogeneous surfaces, where f1 and f2 are
  the fractional surface areas occupied by these
  two species

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Experimental development

• Lighting
• Magnification
• Picture Clarity
• Surface consistency

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General Exploration
To determine the focus of our research we
experimented with different physical variables to
observe what affected the contact angle

• Water mixed with soap
• Droplet on a surface that was already wet

• Hot water
• Cold water
• Varying pressure used to prepare the surface

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Time vs. Contact Angle

• We placed a droplet on the prepared surface and
  took pictures at one minute increments to
  measure the change in contact angle

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Results
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Results

• Approximate calculations indicated that nearly
  50 of the droplet should evaporate in an hour
• The contact length did not change noticeably
• The droplet would stick to the surface after a
  short time so strongly that the drop wouldnt
  fall when the surface was inverted

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Contact Angle vs. Surface Tension

• We wanted to observe the change in contact angle
  vs. change in surface tension.
• To vary surface tension we mixed a little ethanol
  in water
• Starting with 100ml pure water and we increased
  the volume of our ethanol/water mixture by .1ml
  and measured the contact angle of a droplet of
  that mixture

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Experimental Refinements

• Along the way we discovered that our contact
  angle depended on the density of our surface
  which tainted our data
• In order to obtain consistent results we designed
  a method of creating a surface with the same
  density throughout
• We also started using a 5 micro liter syringe to
  control the droplet size

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Surface Tension from Volume

• We obtained the surface tension of our
  ethanol/water by using data from The Handbook of
  Physics and Chemistry to interpolate a surface
  vs. volume graph from which we estimated the
  surface tensions of our mixtures

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Results
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Translating the data

• This functionality between surface tension and
  contact angle indicates a change in the surface
  interface between the drop and the surface.
• To measure this change you can use Cassie-Baxter
  relation
• cos ? f1 cos ?1 f2 cos ?2
• where f1 is our surface and f2 is air. Then
  using p as ?2 our equations becomes
• cos ? f1 cos ?1 - f2

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Finding FS

• Then using the fact that
• f1 f2 1
• our equation becomes
• cos ? f1 cos ?1 (1 - f1)
• which can be rearranged to be
• cos ? -1 FS (cos ?E 1)
• where FS f1

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Method of experimentation

• Knowing the contact angle on a rough surface (cos
  ?) given a mixture of ethanol and water, we will
  measure the contact angle of the same mixture on
  a smooth surface (cos ?E) and find FS
• To measure cos ?E we will compress the gel until
  its surface is totally smooth and measure the
  contact angle of a droplet

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Future Experimentation

• Complete experiments to determine FS
• Pressure vs. Contact Angle
• Measuring the contact angle of droplets with
  surface tension above 72 by adding salt in the
  water
• Explore and understand the sticky behavior of the
  surface

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Questions?