Compressible Air Entrapment in High-Speed Drop Impacts on Solid Surfaces

TL;DR

This study uses high-speed imaging and optical interference to analyze how compressed air films form and evolve during high-speed liquid drop impacts on solid surfaces, revealing the dynamics of air entrapment and bubble formation.

Contribution

It provides the first detailed experimental characterization of the formation, pressure, and evolution of entrapped air films during high-speed drop impacts.

Findings

Compressed air films form before liquid contact at high impact velocities.

The pressure in the air film exceeds atmospheric pressure significantly.

The air film expands and contracts, leading to bubble formation.

Abstract

Using high-speed photography coupled with optical interference, we experimentally study the air entrapment during a liquid drop impacting a solid substrate. We observe the formation of a compressed air film before the liquid touches the substrate, with internal pressure considerably higher than the atmospheric value. The degree of compression highly depends on the impact velocity, as explained by balancing the liquid deceleration with the large pressure of compressed air. After contact, the air film expands vertically at the edge, reducing its pressure within a few tens of microseconds and producing a thick rim on the perimeter. This thick-rimmed air film subsequently contracts into an air bubble, governed by the complex interaction between surface tension, inertia and viscous drag. Such a process is universally observed for impacts above a few centimeters high.