for γ γ w the surface tension of water, γ γ s , the (lower) surface tension of the surfactant-covered water surface, μ μ the viscosity of water, and ρ ρ the mass density of water. For ( γ γ w − − γ γ s ) ≈ ≈ 10 − − 2 N/m, i.e., of order tens of per cent reduction in surface tension of water, and as for water ( μ μ ρ ρ ) ∼ ∼ 1 N m s, we obtain the second equality above. This gives speeds that decrease as surfactant-covered region grows, but are of order cms/s to mm/s.