Measuring and interpreting contact angles: a complex issue

Abstract

Low-rate dynamic contact angles of 30 liquids on a FC-725-coated wafer surface were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). Surprisingly, results indicate that FC-725 behaves differently in some respects from what one would expect for non-polar surfaces: only nine liquids yield essentially constant contact angles whereas the others show slip/stick contact angle behaviour. In the worst case, the contact angle increases from ca 50 to 160° at essentially constant three-phase contact radius. These angles should be disregarded for the interpretation in terms of surface energetics since there is no guarantee that Young's equation is applicable. If one employs a conventional goniometer-sessile drop technique, such contact angle behaviour cannot be easily seen in all cases. These results indicate that the claim from van Oss et al. [Langmuir 4 (1988) 884] that liquids with the same contact angles do not have the same surface tensions is misleading. If the meaningful contact angles are plotted as the liquid–vapour surface tension times cosine of the contact angle versus the liquid–vapour surface tension, that is, γ lv cos θ versus γ lv, a smooth curve emerges. Thus, intermolecular forces (or surface tension components) do not have an additional and independent effect on the contact angles, in good agreement with the results from other studies on non-polar and polar polymers.