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

DOMENIC MCARTHUR, KERRY O’BRIEN, VARGHESE

MATHAI, University of Massachusetts, Amherst — We study water

entry of steel spheres in the presence of an ultrasoft, thin elastic mem-

brane, a few hundred microns in thickness, floating on a free surface.

The presence of the 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 be-

fore 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 mem-

brane relaxes and recoils while pivoting on the sphere. Remarkably, 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.