2.2.1. Contact Angle

The measurement techniques of contact angle can be classified into two main groups, the direct optical methods and the indirect force methods. The most common direct methods are the sessile drop, the captive bubble and the tilting plate (Figure 1). Indirect methods include tensiometer and geometric analysis of the shape of a meniscus. Calculations based on measured contact angle values yield are an important parameter of the solid surface tension, which quantifies the wetting characteristics of a solid material surface [24].

Common methods of the contact angle measurements: (a) sessile drop; (b) captive bubble; (c) tilting plate. θ is a contact angle.

The most popular technique of contact angle measurement is a direct measurement of the tangent angle at the three-phase contact point on a sessile drop profile (Figure 1a). This optical method needs small amounts of liquid and small surface of solid. The contact angle of a liquid drop on an ideal horizontal solid surface is determined by the mechanical equilibrium of the drop under the action of three interfacial tensions of liquid (Figure 2).

A liquid droplet remaining in equilibrium state with a plane solid surface: γSL - free surface energy between solid and liquid; γSV - free surface energy between solid and vapor; γLV - free surface energy between liquid and vapor.

The Formula (1) is usually referred as Young’s equation, where γ_LV, γ_SV, γ_SL represent the liquid–vapor, solid–vapor and solid–liquid interfacial tensions, respectively, and θ is measured contact angle of liquid.

The Young’s equation applied to a concrete liquid–solid system, describes three thermodynamic parameters and the contact angle. However, there exist many states of a drops on a surface solid, and the observed contact angles are usually not equal to θ. The liquid migrates on surface and wet the new area of surface of the solid, so the phenomenon of wetting is not a static state.