ABSTRACT

A knowledge of the contact angle of a liquid on a solid surface is important in understanding wetting and adhesion [1,2]. Changes in the equilibrium contact angle are used as indications of changes in the wettability of surfaces and are related by Young’s law, cos# = (ySv - Xsü/lAv, where the γί} are the interfacial tensions. Optical observation of the profile of a droplet on a surface is an important technique for accurate determination of the contact angle. However, materials often arise in the form of a fiber and this presents quite specific problems in measuring the contact angle from the observed profile. On a flat surface, a droplet of liquid characterized by a spreading power S = ysv — 7 s l — Kl v ^ 0 will spread to form a thin film so that no macroscopic shape exists. On a fiber of the same material, a droplet may

*To whom correspondence should be addressed. E-mail: glen.mchale@ntu.ac.uk

exhibit either a clam-shell or a barrel-type conformation depending on its volume (Fig. 1) [3, 4], Moreover, it is consistent with a vanishing equilibrium contact angle for the barrel-type droplet on a fiber to have a macroscopic shape rather than simply forming a thin sheathing film [2, 5-7], This type of droplet requires an inversion of curvature in the profile close to the fiber surface. The excess pressure across a curved surface is given by the Laplace law A P = Ylw(1/R\ + l//?2), where R\ and R2 are the two principal radii of curvature. This excess pressure can be reduced by either increasing the two radii of curvature or introducing a relative sign between the radii. Film formation on a flat surface requires both radii to become large and this is consistent with conservation of the liquid volume. However, for barrel-type droplets on a fiber, volume conservation necessarily means that increasing one radius of curvature involves a reduction in the other. A limit is set on the reduction in the radius of curvature by the radius of the fiber. Thus, a reduction in the excess pressure can be achieved whilst maintaining finite absolute values for the radii of curvature provided that there is a change in curvature, i.e. an inflection, in the droplet profile.