1. The Hydrophobic Characteristics of Aerosol Gel

2. Research Summer 06 Goals Applications
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

3. Aerosol gel characteristics
Fractal Aggregate
Density approximately 2.0 mg/cc
High surface area
Electrically conductive
Hydrophobic

4. Definitions
A “hydrophobic” surface is a surface that the contact angle of a water droplet on it > 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)

5. Theory of hydrophobicity
Ideas of surface tension/surface energy between phases yield the Young relation, assuming a smooth and chemically homogeneous surface
cos θE = (γSV–γSL)/γLV

6. Refining Theory (a.k.a. Taking reality into account)
Wenzel’s relation was developed as an attempt to understand the affect of rough surfaces on hydrophobicity
cos θ* = r cos θE

7. More refining cos θ* = f1cosθ1+f2cosθ2
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

8. Experimental development
Lighting
Magnification
Picture Clarity
Surface consistency

9. General Exploration Water mixed with soap Cold water
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

10. 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

11. Results

12. 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 wouldn’t fall when the surface was inverted

13. 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

14. 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

15. 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

17. Results

18. 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 θ* = f1 cos θ1 + f2 cos θ2
where f1 is our surface and f2 is air. Then using π as θ2 our equations becomes
cos θ* = f1 cos θ1 - f2

19. Finding ΦS … Then using the fact that f1 + f2 = 1 our equation becomes
cos θ* = f1 cos θ1 – (1 - f1)
which can be rearranged to be
cos θ* = -1 + ΦS (cos θE + 1)
where ΦS = f1

20. Method of experimentation
Knowing the contact angle on a rough surface (cos θ*) given a mixture of ethanol and water, we will measure the contact angle of the same mixture on a smooth surface (cos θE) and find ΦS
To measure cos θE we will compress the gel until it’s surface is totally smooth and measure the contact angle of a droplet

21. Future Experimentation
Complete experiments to determine ΦS
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

22. Questions?