Water entry of steel spheres in the presence of an ultra-thin elastic membrane

We study water entry of steel spheres in the presence of an ultrasoft, thin elastic membrane of thickness of a few hundred microns floating on a free surface. The presence of a thin elastic layer of negligible inertia establishes an air-water interface that can sustain a strain-dependent surface tension. As the sphere impacts the free surface, we observe the formation of a dynamically elongating and propagating deformation front surrounding the impacting body (which elastically stores the energy in the membrane material), and an outer zone where radial wrinkles gradually develop before the membrane eventually gets pulled down by the descending sphere. During the first stage of impact and elongation, the outer zone beyond the deformation front of the membrane is unperturbed by the impact and penetration of the sphere. Following this stage, the stretched membrane relaxes and recoils while pivoting on the sphere. We observe regimes when the ultrathin floating film is able to elastically absorb all of the kinetic energy of the impactor and dissipate it into the surrounding fluid motion. We map out the water entry behavior over a range of dimensionless groups including the Weber number and Froude number as well as the membrane parameter, such as the elasto-inertial number.