Partial wetting (θ=90°)
Contact angle θ90.0°
Radius of curvature R—
Base radius r—
Height h—
ΔP = 2γ/R (Laplace)—
Contact angle hysteresisθ_adv−θ_rec
cos θ_Y = (γ_SV−γ_SL)/γ_LV—
Wenzel roughness effect—
Spreading coefficient S—
Young's Equation (1805):
cos θ = (γSV − γSL) / γLV
Young-Laplace pressure: ΔP = γ(1/R₁ + 1/R₂) = 2γ/R for a sphere
Regimes:
• θ < 90°: hydrophilic (partial wetting)
• θ ≈ 0°: complete wetting (S > 0)
• θ > 90°: hydrophobic
• θ > 150°: superhydrophobic (lotus effect)
Spreading coefficient: S = γSV − γSL − γLV
S > 0 → spontaneous spreading
Cassie-Baxter: air pockets on rough surface → apparent θ increases.
Wenzel: roughness amplifies wettability.