Studies of Surface Species by the Adhesion Method

In the first experiments, the adhesion of the PDMS rubber to itself was measured to give a work of adhesion of 44 mJ significantly smaller than the 

The wetting liquid caused the black spot to decrease whereas the nonwetting liquid caused the black spot to increase in size. For example water, which does not wet PDMS rubber but has a contact angle of 102° as shown in Fig. 6.5(b), gave a work of adhesion of 74mJin . In this case the liquid makes the 

When the contact angle is 90° and the liquid droplet forms a little hemisphere on the surface, then no work is done by the wetting, and the work of adhesion in liquid is the same as in vapor. But if the liquid wets the surface then adhesion must be reduced. This is equivalent to Young s original theory of 1805, which he did not express in symbols but only in words. All the results above were consistent with this equation. Thus the second law of adhesion is true for wetting liquids. 

To check this law out more rigorously, Chaudhury and Whitesides made 11 different mixtures of methanol and water, ranging from 5% methanol to 100%, and used them to verify Young s equation. First they measured the surface tension Ylv of each mixture, then they measured the contact angle 6 of each mixture on a flat surface of the PDMS rubber. This enabled them to calculate the term2yLv cos in the equation of Fig. 6.7. Plotting this as the vertical axis in Fig. 6.8 allowed a comparison with the work of adhesion of the rubber ITsi. measured in the 11 methanol/water mixtures. 

Then Chaudhury and Whitesides found a way of modifying their silicone rubber surface to change its chemical character. The PDMS polymer was exposed to an oxygen plasma for a short period, as shown in Fig. 6.9, creating a thin layer of silica on the surface, about 3 nm thick. By treating this silica layer with molecules of siloxane, single molecular layers, i.e. monolayers of particular structures, could be formed at the rubber surface. 

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