Soap Film Dynamics

Physics: Soap film colors arise from thin-film interference: constructive at 2nt = mλ, where n ≈ 1.34 is the film refractive index, t is thickness, m is integer. Surface tension γ ≈ 0.04 N/m for soap solution (pure water: 0.073 N/m). Drainage proceeds via gravity and Marangoni flow (surfactant gradients drive flow toward thinner regions). The black film (~10 nm thick) appears just before rupture — at this thickness only van der Waals forces stabilize it. Film lifetime depends critically on surfactant type and concentration.