Maximum spreading radius. The droplet impacts the surface with a hydrostatic (inertial) pressure ρU 2 I which is resisted by the Laplace pressure γ/R. The extent of droplet deformation and spreading is therefore determined by how U I compares to the inertial-capillary speed U γ = ! γ/ρR, more precisely to the normalized, non-dimensional impact velocity Ũ = U I /U γ ; the droplet becomes highly deformed when Ũ ≫ 1. Note that Ũ 2 is equivalent to the Weber number W e often used in the literature.