Contact Angle Hysteresis

Physics: Hysteresis Δθ = θ_A − θ_R (advancing minus receding angle). Wenzel state: cosθ_W = r·cosθ_Y (roughness enhances wetting tendency). Cassie-Baxter state: cosθ_CB = φ_s·cosθ_Y + φ_s − 1 (air pockets reduce wetting; φ_s is solid area fraction). Lotus effect: superhydrophobic θ > 150° via dual-scale roughness (micro + nano), Δθ < 5° so droplets roll off, self-cleaning. Droplet slides when mg·sinα > γ(cosθ_R − cosθ_A)·w (width of contact line).