Page 53 - Engineered Interfaces in Fiber Reinforced Composites

36 Engineered interfaces in fiber reinforced composites

2.3.11.3. Contact angle on a cylindrical surface
The techniques for measuring the contact angle of droplets on planar surfaces
have been discussed above. The measurement of the contact angle for wettability of
a cylindrical surface of the order of 10 pm in diameter as for advanced fibers
requires a more sophisticated approach than is needed for a simple planar surface. A
widely used method is based on the Wilhelmy balance method by use of gravimetry
(Kaelble et al., 1974; Hammer and Drzal, 1980). The contact angle may be
determined by measuring the force required to immerse or remove a single fiber
from a liquid of known surface tension at constant velocity. Hammer and Drzal
(1980) determined the polar and dispersive components of a small diameter graphite
fiber by measuring the contact angle. A single fiber was immersed in a liquid and the
force, F, exerted by the liquid on the fiber was measured with microbalance due to
the wetting of the fiber. The force, F, is related to the surface tension of the liquid,
yLv, by the equation
F = yLvnd COS e , (2.15)

where d is the fiber diameter. The polar and dispersive components of the fiber
surface tension, (7: and 7:)’ are determined based on the following equation:

(2.16)

A plot of yL (1 + cos e)/2(yt)1/2 versus (y;/yt)li2 will yield a straight line with the
slope and intercept providing a solution for the components yg and $, respectively,
for the fiber.
A simple and direct method of contact angle measurement has also been proposed
(Yamaki and Katayama, 1975; Carroll, 1976) by observing the shape of the liquid
droplet attached to a single fiber, the so-called ‘droplet aspect ratio method’. The
liquid is assumed to form a symmetrical droplet about the fiber axis as shown
schematically in Fig. 2.21. Neglecting the effect of gravity, the droplet shape can be
defined by the following expression:

(2.17)
J5 = 2b F(4,K) + nE(4,41 I
where the parameters are:
e
L=-, (2.18)
XI

(2.19)

(2.20)