Static Contact Angle
The contact angle, θ, is the angle formed by a liquid at the three phase boundary where the liquid, gas, and solid intersect. The contact angle depends on the interfacial tensions between the gas & liquid, liquid & solid, and gas & solid. Young’s Relation expresses the contact angle analytically.
where
- = Interfacial tension between the solid and gas
- = Interfacial tension between the solid and liquid
- = Interfacial tension between the liquid and gas
Contact angle goniometers measure a droplet’s contact angle by assuming the droplet fits the geometry of a sphere, ellipsoid, or the Young–Laplace relation.
Another perspective that describes contact angles uses cohesion vs. adhesion. Cohesion is the force between the liquid molecules which hold the liquid together. Adhesion is the force between the liquid molecules and the solid molecules. The contact angle is a quantitative measure that tells the user the ratio of cohesion vs. adhesion. If the contact angle is near zero, meaning the liquid droplet spreads completely on the solid surface, adhesive forces are dominating. If the contact angle is very high, meaning the liquid droplet beads up on the solid surface as water does on a freshly waxed car, cohesive forces are dominating.
Read more about this topic: Goniometer, Applications, Surface Science
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