Mep. Walinder et G. Strom, Measurement of wood wettability by the Wilhelmy method - Part 2. Determination of apparent contact angles, HOLZFORSCH, 55(1), 2001, pp. 33-41
This work focuses on the determination of apparent contact angles on wood b
y the Wilhelmy method. In this method, the force acting on an object is mea
sured during a test cycle involving immersion in and withdrawal from a prob
e liquid. Fresh and aged veneers of extracted and non-extracted heart- and
sapwood of pine were investigated. The results indicate that wicking of the
probe liquids. into and along the porous wood veneers, occurs during the t
est cycles and that this strongly affects the determination of contact angl
es. It is suggested that two different wicking phenomena occur. First, when
the veneer contacts the liquid, an instantaneous 'initial wicking' occurs.
It is suggested that this initial wicking is influenced primarily by the l
iquid density and structural properties of the specimen (such as porosity a
nd sur face roughness), and not by surface energetics. An initial wicking c
onstant was therefore estimated for the different veneer samples based on m
easurements in octane. Second, after the initial wicking, a continuing 'sec
ondary wicking' is observed. In some cases. this may result in zero contact
angle after a certain immersion depth. Contact angles should, therefore, b
e estimated from the initial part of the immersion, where the secondary wic
king can be neglected. This may also reduce any contamination of the probe
liquids by extractives. The Wilhelmy method stems to be a valuable tool for
estimating the wetting properties of wood, permitting reproducible measure
ments of apparent contact angles provided that there is efficient control o
f wicking and contamination effects.