Past exam of the soft condensed matter course of the University of Cambridge / 2021 / Section B / 5 / a

A hydrophobic surface is poorly wetted by water: on a smooth surface its equilibrium contact angle, measured through the liquid, exceeds $90^{\circ }$. A drop therefore tends to bead up, limiting its contact area. Hydrophobicity does not mean that the surface exerts a mysterious repulsive force on every water molecule.

The waxy coating has a largely nonpolar exterior. Water molecules form favorable hydrogen bonds with one another, whereas a hydrocarbon-rich wax surface offers few suitable hydrogen bond donors or hydrogen bond acceptors. Water–surface adhesion is weak relative to water's cohesion. The drop shape minimizes the combined solid–air, solid–water and water–air interfacial free energies, not merely the liquid's exposed area.

In terms of Young's equation, ${\gamma }_{SV}-{\gamma }_{SL}={\gamma }_{LV}\cos \theta$. A contact angle greater than $90^{\circ }$ corresponds to ${\gamma }_{SL}>{\gamma }_{SV}$: replacing dry surface by wetted surface has an interfacial cost. The balancing liquid interface then favors a small wetted patch. Microscopic leaf texture can amplify the apparent hydrophobicity, especially when it traps air under the drop as a Cassie–Baxter state; the chemistry of a smooth wax layer alone does not guarantee an extremely large apparent angle.