The Hydrophobic Characteristics of Aerosol Gel

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

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

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)

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

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

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

Experimental development Lighting Magnification Picture Clarity Surface consistency

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

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

Results

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

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

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

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

Results

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

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

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

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

Questions?