Approach and Coalescence of Liquid Drops in Air

TL;DR

This paper investigates the initial stages of liquid drop coalescence in air, revealing three distinct regimes and the influence of approach speed and ambient gas on the process, using ultrafast measurement techniques.

Contribution

It provides a detailed experimental analysis of early coalescence dynamics, supporting a three-regime model and highlighting the role of ambient gas and approach speed.

Findings

Identification of three coalescence regimes.

Threshold approach-speed for drop deformation.

Support for the inertially limited viscous regime.

Abstract

The coalescence of liquid drops has conventionally been thought to have just two regimes when the drops are brought together slowly in vacuum or air: a viscous regime corresponding to the Stokes-flow limit and a later inertially-dominated regime. Recent work [Proc. Natl. Acad. Sci. 109, 6857 (2012)] found that the Stokes-flow limit cannot be reached in the early moments of coalescence, because the inertia of the drops cannot be neglected then. Instead, the drops are described by an "inertially limited viscous" regime, where surface tension, inertia, and viscous forces all balance. The dynamics continue in this regime until either viscosity or inertia dominate on their own. I use an ultrafast electrical method and high-speed imaging to provide a detailed description of coalescence near the moment of contact for drops that approach at low speed and coalesce as undeformed spheres. These measurements support a description of coalescence having three regimes. Signatures both before and after contact identify a threshold approach-speed for deformation of the drops by the ambient gas.