About Contact Angles

The angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid is called the angle of contact (θ).

When θ is an obtuse angle (θ > 90°), liquid molecules are strongly attracted to themselves and weakly to the solid (like water on a waxy surface). When θ is an acute angle (θ < 90°), liquid molecules are strongly attracted to the solid (like water on glass).

The equilibrium condition is given by: S_acosθ + S_sl = S_sa, where S_a is liquid-air surface tension, S_sl is solid-liquid interfacial tension, and S_sa is solid-air surface tension.