Surface tension, wetting and capillarity
A liquid surface behaves as an energetic boundary because molecules at the surface experience an unbalanced inward attraction.
1. Surface tension
Inside a liquid, intermolecular attractions act in all directions and largely balance. At the surface there are fewer neighbouring molecules above, so the resultant attraction is inward. Creating more surface requires work. Surface tension γ is force per unit length, γ = F/L, measured in N/m; it is also surface energy per unit area in J/m². A liquid therefore tends to reduce its surface area, which helps small droplets become nearly spherical. Surface tension usually decreases as temperature rises and can change strongly when a surfactant is added.
2. Wetting and contact angle
Cohesion is attraction among liquid molecules; adhesion is attraction between the liquid and another material. The contact angle θ is measured through the liquid where liquid, solid and gas meet. When adhesion is relatively strong, θ is less than 90° and the liquid wets the surface, often forming a concave meniscus. When cohesion dominates, θ is greater than 90°, wetting is poor and the meniscus is convex.