Contact angle measurement and contact angle interpretation

• Published in: Advances in colloid and interface science Vol. 81; no. 3; pp. 167 - 249
• Main Authors: Kwok, D.Y., Neumann, A.W.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 07-09-1999; Elsevier
• Subjects: Chemistry; Combining rule; Contact angle; Drop shape method; Exact sciences and technology; General and physical chemistry; Solid surface tension; Solid-liquid interface; Surface physical chemistry; Surface tension component; Solid surface tension; Contact angle; Surface tension component; Combining rule; Drop shape method; Liquid solid interface; Surface properties; Surface tension; Review; Measurement method; Sessile drop method; Solid; Drop
• ISSN: 0001-8686; 1873-3727
• DOI: 10.1016/S0001-8686(98)00087-6

Recent progress in the correlation of contact angles with solid surface tensions are summarized. The measurements of meaningful contact angles in terms of surface energetics are also discussed. It is shown that the controversy with respect to measurement and interpretation of contact angles are due to the fact that some (or all) of the assumptions made in all energetic approaches are violated when contact angles are measured and processed. For a large number of polar and non-polar liquids on different solid surfaces, the liquid–vapor surface tension times cosine of the contact angle, γ
lv
cosθ, is shown to depend only on the liquid–vapor surface tension γ
lv
, and the solid–vapor surface tension γ
sv
when the appropriate experimental techniques and procedures are used. Equations which follow these experimental patterns and which allow the determination of solid surface tensions from contact angles are discussed. Universality of these experimental contact angle patterns is illustrated; other reasons which may cause data to deviate from the patterns slightly are discussed. It is found that surface tension component approaches do not reflect physical reality. Assuming the fact that solid surface tension is constant for one and the same solid surface, experimental contact angle patterns are employed to deduce a functional relationship to be used in conjunction with Young's equation for determining solid surface tensions. The explicit form of such a relation is obtained by modifying Berthelot's rule together with experimental data; essentially constant solid surface tension values are obtained, independent of liquid surface tension and molecular structure. A new combining rule is also derived based on an expression similar to one used in molecular theory; such a combining rule should allow a better understanding of the molecular interactions between unlike solid–liquid pairs from like pairs. Existing static contact angles for 34 different types of solid surfaces from Zisman et al. are evaluated in terms of their solid surface tensions using experimental contact angle patterns. A FORTRAN computer program has been implemented to automate these procedures. It is found that literature contact angles do not have to be discarded completely; they can be used to determine solid surface tensions, with caution. The surface tensions for the 34 solid surfaces from Zisman et al. are also reported.