Time-resolved imaging of a compressible air disc under a drop impacting on a solid surface

Abstract

When a drop impacts on a solid surface, its rapid deceleration is cushioned by a thin layer of air, which leads to the entrapment of a bubble under its center. For large impact velocities the lubrication pressure in this air layer becomes large enough to compress the air. Herein we use highspeed interferometry, with 200 ns time-resolution, to directly observe the thickness evolution of the air-layer during the entire bubble entrapment process. The initial disc radius and thickness shows excellent agreement with available theoretical models, based on adiabatic compression. For the largest impact velocities the air is compressed by as much as a factor of 14. Immediately following the contact, the air disc shows rapid vertical expansion. The radial speed of the surface minima just before contact, can reach 50 times the impact velocity of the drop.

Probing the nanoscale with high-speed interferometry of an impacting drop

“…Our results also confirmed the theoretical predictions of the initial diameter of the air-disc, as well as the radial velocity of the kink in the bottom shape of the drop (12) .…”

“…This contraction into the central bubble is accurately modeled with a capillary-inertial scaling, for the low-viscosity case (23)(24)(25) . Our time-resolved measurements of the air-disc size and initial thickness have essentially verified the theoretical treatment of this problem (12) , where the basic balance is between viscous lubrication pressure within the gas in the thin air-layer and the inertia of the bottom section of the drop. This introduces a Stokes number, = / , as the controlling non-dimensional parameter.…”

“…Optical setups used for the interferometric imaging. (a) Monochromatic light is either shone from the bottom (Light 2) through a half-silvered mirror, for reflective interferometry, or from the top (Light 1) for transmission interferometry (12) . (b) Twocolor interferometry setup, used in Langley et al (15) .…”

“…(b) Twocolor interferometry setup, used in Langley et al (15) . (c,d) Interference fringes under the impacting drop, from transmitted (c) and reflective (d) interferometry with monochromatic light (12) . Scale bars are both 200 µm long.…”

“…Our time-resolved imaging has revealed new aspects of the air-layer dynamics (12) , as the bottom of the drop is deformed into a dimple, which makes first contact with the solid not at a point, but rather along a ring which entraps the air-film. Furthermore, at high impact velocities, ∼3 m/s, there is significant compression of the air.…”