The data indicate that with the more intense plasma treatment, the hydrophilicity
of the surface is more stable over a period of a week, however, the high standard
deviation after 1 week exposure to air for 100 W sample implies that the surface has areas
of homogeneity that are rather extreme. Some areas had contact angles as low as 250
while others had angles -800. The stability of the surface is possibly due to a silica-like
crust as mentioned above, and the areas of hydrophobicity are due to low MW oligomers
migrating through cracks.
Dynamic Contact Angle
Dynamic contact angles are taken by advancing or removing a liquid interface on
a surface. This can be accomplished with a variation of the sessile drop method by either
adding or subtracting fluid, or by tilting the plate and measuring the angle. Another
technique is the Wilhelmy plate method, where a film of material is dipped in a liquid
and the force on the plate is measured. The force is related to the surface tension of the
liquid by the equation:
F = y Pcos 0
where P is the perimeter of the plate. By measuring the force and perimeter of a sample
in a known liquid, the contact angle can be determined for both advancing (inserting) and
receding (withdrawing) contact angles.
Dynamic contact angle data was taken on unmodified silicone and the linear non-
functionalized additives. Functionalized oligomers were not available at the time of
examination and the method of production had yet to be determined. In addition, samples
with both smooth and textured areas were examined. Figure 3.9 is a representative force-
distance curve taken from DCA data. The lower linear portion of the curve represents the