Extending the large bubble entrapment regime and modelling the mechanics

When a drop of liquid falls on a liquid pool it becomes partially or completely coalesced in the liquid, or splashes. Often with coalescence, air bubble entrapment occurs. A natural phenomenon, such as oceanic mist formation, involves bubble entrapment, which is also a feature of aeration dynamics in many industries (e.g. pharmaceuticals).

Researchers in 2013 experimentally observed large bubble entrapment outside of the small, traditionally classified region on the velocity versus drop-diameter (V-D) map. However, natural droplet dynamics can complicate the experimental boundary outlining of large bubble entrapment. Recently, researchers in computational fluid dynamics from India and Taiwan used numerical simulations involving marker and cell algorithm, the HYPRE multigrid solver and ENO scheme, to re-define and extend the region of large bubble entrapment. They report their findings in Physics of Fluids.

The simulation was validated against existing experimental results, accurately capturing inward and upward jets, as well as the sequential small bubble formation that sometimes follows large bubble entrapment.

The highly refined mesh model captures large bubble entrapment for prolate—vertically elongated—bubbles. Unlike flatter droplets, prolate droplets induce vortex-ring detachment from the surface and penetration into the liquid, deforming the interface to produce an elongated roll jet. This then collapses on the central axis to entrap the large bubble.

The researchers observed that entrapment of a large bubble is restricted if surface conditions lead to ejecta sheet development merged with impact induced lamella, that subsequently widens the crater mouth and suppresses vortex-ring formation.

The team at the Indian Institute of Technology, Guwahati led by Gautam Biswas is applying their simulations to examine how the impact of serial droplets affects bubble entrapment.

Source: “The regime of large bubble entrapment during a single drop impact on a liquid pool,” by Hiranya Deka, Bahni Ray, Gautam Biswas, Amaresh Dalal, Pei-Hsun Tsai, and An-Bang Wang, Physics of Fluids (2017). The article can be accessed at https://doi.org/10.1063/1.4992124 .