Tunneling of micron-sized droplets through soap films

When a micron-sized water droplet impacts on a freely suspended soap film with speed vᵢ, there exists a critical impact velocity of penetration vC. Droplets with vᵢ<vC merge and flow with the film after impacts, whereas droplets with vᵢ>vC tunnel through it. In all cases, the film remains intact despite the fact that the droplet radius (R₀=26 μm) is much greater than the film thickness (0<h10 μm). The critical velocity vC is measured as a function of h, and interestingly vC approaches an asymptotic value vC0520 cm/s in the limit h→0. This indicates that in addition to an inertial effect, a deformation or stretching energy of the film is required for penetration. Quantitatively, we found that this deformation energy corresponds to the creation of ~14 times of the cross-sectional area of the droplet (14πR₀²) or a critical Weber number WeC(≡2ρwvC0²R₀/σ)44, where ρw and σ are, respectively, the density and the surface tension of water.