Abstract
Contact angle measurement has widely been used to characterize the properties of solid surfaces and study liquid–surface interactions. It has been known for some time that, while the measurement itself is deceptively simple, the interpretation is not straightforward and can be very complex. Correlations between contact angle data (static contact angle θ, advancing and receding contact angle θA and θR, hysteresis (θA−θR), and sliding angle α) and surface wettability and adhesion are at times confusing. In an attempt to find out what surface properties contact angles are measuring, Samuel, Zhao, and Law systematically measure the wetting and adhesion forces between water and 20 surfaces and correlate them with contact angle data. The surface properties of the 20 surfaces vary from hydrophilic to hydrophobic to superhydrophobic, and their morphology varies from atomic smooth to homogeneously rough in the nano- and micron scale. Based on the good correlations found between θA and the wetting force and θR with the adhesion force, it was concluded that θA and θR are measures of surface wettability and adhesion, respectively. Since sliding angle α is a measure of drop mobility, it is recommended that surface should be characterized by their wettability, adhesion and stickiness using θA, θR, and α, respectively. As for contact angle hysteresis, the analysis suggests that it is a measure of the difference in liquid–surface interfacial tension during advancing and during receding. The use of the basic concepts described in this chapter to comprehend properties displayed by some recently reported unconventional surfaces is discussed. These unconventional surfaces are surfaces with very large contact angles but very sticky or with small contact angles and very slippery.
Published Version
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