Contact angles and surface energetics

grantor: University of TorontoLow-rate dynamic contact angles of a large number of liquids are measured on various solid surfaces by Axisymmetric Drop Shape Analysis - Profile (ADSA-P). It is found that not all experimental contact angles can be used for energetic calculations: In a specific case, slip and stick contact angle behaviour occurs where the contact angle fluctuates by as much as 35°. Thus, circumspection is necessary in the decision whether or not the experimental contact angles can be used to surface energetics in conjunction with Young's equation and whether the solid-liquid systems violate the basic assumptions made in all contact angle approaches. It is shown that if one omits the inconclusive contact angle measurements, the liquid-vapour surface tension times cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e. glncosq depends only on gln for a given solid surface (or solid surface tension). Changing the solid surface (and hence gsn ) shifts the curves in a very regular manner. Thus, glncosq depends only on gln and gsn . Intermolecular forces do not have an additional and independent effect on the contact angles. Because of Young's equation, the solid-liquid surface tension gsl can be expressed as a function of only gln and gsn . Goniometer and ADSA-P contact angle measurements are also compared: they are shown to be essentially identical for solid-liquid systems which have constant contact angles. In the specific case of the slip/stick of the three-phase contact line using a goniometer technique, the observed static advancing angle corresponds to the maximum angle of the entire slip/stick behaviour, as registered by the automated ADSA-P technique. Thus, conventional goniometer measurements may produce a mixture of meaningful and meaningless contact angles, with no criteria to distinguish between the two. This provides partial explanation of the contact angle controversy in the literature. A large amount of dynamic contact angle data is generated. An equation which follows these experimental patterns and which allows the determination of solid surface tensions is discussed.Ph.D