Characteristic micro- and nano- surface morphology combined with low surface energy chemical functionality allows for water droplets to roll and bounce freely on the surface.

The surface texture can greatly reduce the solid-liquid surface contact area and leads to an apparent contact angle: θ* Surfaces with high θ* ( 150° ~ 180°) and low water contact angle hysteresis (10°) are superhydrophobic. Superhydrofobicity can be explained by two independently developed models: the Wenzel model and the Cassie-Baxter model.

stick_surface cos θ CB* = φ(cos θ + 1) – 1 Φ: fraction of solid liquid contact

Metastable Cassie State: If a Wenzel state is less energetic any perturbation of the Cassie Drop Can provoke its perturbation to the Wenzel state. Energy barrier: E = (γ SL − γ SA )(r − 1)=−γ (r − 1) cos θ Can be overcome by throwing the drop on the surface, pressing or by vibration. This energy barrier in general too large to be overcome by thermal energy.

The smaller the drop, the larger the δ is. When it becomes of the order of the pillar height, h, a solid/liquid contact can nucleate on the substrate and propagate. Critical radius for a Cassie drop scaling: R ∗ ∼ p 2 /h. The radius R ∗ can be much larger than p if h < p. This can be achieved by making h large or by reducing both p and h.

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