A drop of a liquid was spilled on a solid surface under air. The liquid droplet changed its shape on the solid surface until an equilibrium state was established.

(a) Write the equation describing the balance of the interfacial tensions at the equilibrium for the triple interface point solid/liquid/gas. Provide a sketch.

(b) Describe different degrees of wetting, which can be observed for the system solid/liquid/gas. Comment on the actual contact angle for each case.

(c) What would happen if a drop of water ( = 72 mN m-1 ) will be spilled over a solid surface of SiO2 ( = 307 mN m-1 )?

(d) If sodium dodecyl sulphate were to be added to a water droplet spilled on solid SiO2, would the degree of wetting change? Justify!