title: Tunneling of micron-sized droplets through soap films.
abstract: When a micron-sized water droplet impacts on a freely suspended soap film with speed v(i), there exists a critical impact velocity of penetration v(C). Droplets with v(i)<v(C) merge and flow with the film after impacts, whereas droplets with v(i)>v(C) tunnel through it. In all cases, the film remains intact despite the fact that the droplet radius (R_{0}=26 μm) is much greater than the film thickness (0<h≲10 μm) . The critical velocity v(C) is measured as a function of h , and interestingly v(C) approaches an asymptotic value v(C0)≃520 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 (14πR(0)(2)) or a critical Weber number We(C)}(≡2ρ(w) v(C0)(2) R(0)/σ)≃44 , where ρ(w) and σ are, respectively, the density and the surface tension of water.
authors.affiliation: Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA.
authors.forename: Ildoo
authors.initials: I
authors.lastname: Kim
authors.forename: X L
authors.initials: XL
authors.lastname: Wu