[Terminology] Contact angle and Young's equation
Definition: the angle which is from solid-liquid interface by liquid phase to gas-liquid (or liquid-liquid) interface, at the junction of the three phases of solid, liquid, gas (or liquid). Expressed as θ. As shown in Figure 1.
Figure 1 Contact angle schematic diagram
θ -- contact angle; S -- solid phase; L -- liquid; G -- second fluid phase (typical for gaseous phase); γsl -- solid-liquid interfacial tension; γsg -- solid phase to second fluid phase (typical for gaseous phase) interfacial tension; γlg -- liquid to second fluid phase (typical for gaseous phase) interfacial tension.
θ and γsl, γsg, γlg have the following relationship:
This is known as Young's equation. T. Young firstly proposed in 1805.
Further, spreading coefficient S and contact angle θ have the following relationship:
S = γlg (cosθ- 1)
Obviously, the size of contact angle is a good wetting standard. Contact angle is smaller, wetting is easier. Traditionally, θ = 90° is standard for determination of whether to wetting. θ > 90°, non-wetting; θ < 90°, wetting; θ = 0 or non-exist, spreading.