Contact angles in controlled atmospheres: Some theoretical considerations

Abstract

Contact angle measurements in controlled atmospheres must take into account the effect of residual gas molecules in the less-than-perfect vacuum that is encountered experimentally and of the gas molecules formed by vaporization of the liquid component. The possibility of measuring the contact angle of a non-vaporizing liquid has been considered. In an experiment with a low-melting lead bismuth alloy on glass, the contact angle at 142°C was 136°. Calculations based on the Fowkes theory of dispersion forces indicate that the contact angle should not exceed 141°. In contrast to the reasonable agreement with the Fowkes theory, the observed angle is lower than would be expected from the Yarnold-Mason theory as it would apply to a competition between residual gas molecules and metal atoms for the solid surface and the effect of this competition on the contact angle. Finally, the theory of works of adhesion and cohesion is applied to the known variability of the hydrophilicity of graphite surfaces. The known reactivity of adsorbed water with graphite makes it impossible to apply an exact theory, but a limiting value of 53 % hydrophilic surface is indicated for graphite at room temperature. Limits of contact angles of 0° are indicated by the physical meaning of contact angle. If one considers surface energies only, the limits must be placed at 0° (and γ lv cos θ = γ LV) and 180°. However, considerations based on dispersion forces indicate that γ LV cos θ > γ LV should be observable experimentally and contact angles approaching 180° should be impossible of physical existence at equilibrium.