Page 52 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 35

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Fig. 2.20. Wilhelmy slide technique for contact angle measurement. After Adamson (1982).

(2.13)

where a is the capillarity constant. The termination of the meniscus is quite sharp
under proper illumination (unless 8 is small), and h can be measured by means of a
traveling microscope.
2.3.11.2. Contact angle on a rough surface
The foregoing discussion considers the wetting of a smooth planar surface. The
derivation for the contact angle equation given by Eq. (2.11) can be adapted in an
empirical manner to the case of a non-uniform solid surface, whether the surface is
rough (with a roughness index) or is a composite consisting of small patches of
various kinds. Details of this subject have been reviewed by Adamson (1982) and a
summary is given here.
Good (1952) showed that the surface roughness alone may change the advancing
contact angle, Or, on a rough surface, compared with the contact angle, 8, on a
smooth surface of identical surface chemistry. This change in the contact angle can
be expressed by
cos or = rf cos e (2.14)

where rf is the roughness factor, which is the ratio of actual to nominal surface areas
of the solid. If 8 is less than 90°, then roughening will result in a smaller 8, on the
chemically equivalent but rough surface. This will increase the apparent surface
tension of the solid surface, ysv. In contrast, however, if for a smooth surface 0 is
greater than 90°, roughening the surface will increase Or still further, leading to a
decrease in ysv .