Pendant drop formation on a cylindrical polymeric fibre network: predicting the drop shape and its evolution beyond the traditional Laplace surface tension equation

Wettability of fibres is generally characterised by the contact angle of axisymmetrical drops on single fibres. The use of the inflexion angle was introduced as a more precise parameter useful for characterising fibre-liquid interactions. This method does not cover the case of non-symmetrical drops that are usually formed on hydrophobic monofilaments. Therefore in order to study all multi-filament fibre surfaces, the inflexion angle is combined to the pendant drop description of Bashforth and Adams. The Microwetting((R)) device is used for this purpose and, more specifically, to measure the shape evolution of pendant drops hanging under planar surfaces and fibre networks. Several methods are proposed to calculate the characteristic drop shape factor, Bo, as a function of the inflexion angle. The latter relationship is instrumental in obtaining a complete abacus to classify fibre surfaces. The volume to surface relationship gives rough estimations of Bo, whereas a force balance on a drop volume provide a much more accurate series of data. Experiments were conducted with para-aramid Kevlar((R)) fibres and with a glass plane; related statistics are presented. Other surfaces and fibre networks serve to validate the approach. A derived wettability ranking is discussed. (C) 2002 Society of Chemical Industry.