Title: On angled bounce-off impact of a drop impinging on a flowing soap film,
Abstract: Small drops impinging angularly on thin flowing soap films frequently
demonstrate the rare emergence of bulk elastic effects working in-tandem with
the more common-place hydrodynamic interactions. Three collision regimes are
observable: (a) drop piercing through the film, (b) it coalescing with the
flow, and (c) it bouncing off the film surface. During impact, the drop deforms
along with a bulk elastic deformation of the film. For impacts that are
close-to-tangential, the bounce-off regime predominates. We outline a reduced
order analytical framework assuming a deformable drop and a deformable
three-dimensional film, and the idealization invokes a phase-based parametric
study. Angular inclination of the film and the ratio of post and pre impact
drop sizes entail the phase parameters. We also perform experiments with
vertically descending droplets impacting against an inclined soap film, flowing
under constant pressure head. Model predicted phase domain for bounce-off
compares well to our experimental findings. Additionally, the experiments
exhibit momentum transfer to the film in the form of shed vortex dipole, along
with propagation of free surface waves. On consulting prior published work, we
note that for locomotion of water-walking insects using an impulsive action,
the momentum distribution to the shed vortices and waves are both significant,
taking up respectively 2/3-rd and 1/3-rd of the imparted streamwise momentum.
In view of the potentially similar impulse actions, this theory is applied to
the bounce-off examples in our experiments, and the resultant shed vortex
dipole momenta are compared to the momenta computed from particle imaging
velocimetry data. The magnitudes reveal identical order ($10^{-7}$ N$\cdot$s),
suggesting that the bounce-off regime can be tapped as a simple analogue for
interfacial bio-locomotion relying on impulse reactions.