Theory for the coalescence of viscous lenses

TL;DR

This paper develops a theoretical framework for understanding the coalescence dynamics of viscous liquid lenses, revealing self-similar behavior and the influence of boundary conditions, supported by numerical simulations and experiments.

Contribution

It introduces a detailed analysis of viscous liquid lens coalescence using thin-sheet equations, highlighting the effects of boundary conditions and global translation.

Findings

Self-similar bridge growth at leading order

Boundary conditions significantly affect coalescence dynamics

Experimental confirmation of global translation during coalescence

Abstract

Drop coalescence occurs through the rapid growth of a liquid bridge that connects the two drops. At early times after contact, the bridge dynamics is typically self-similar, with details depending on the geometry and viscosity of the liquid. In this paper we analyse the coalescence of two-dimensional viscous drops that float on a quiescent deep pool; such drops are called liquid lenses. The analysis is based on the thin-sheet equations, which were recently shown to accurately capture experiments of liquid lens coalescence. It is found that the bridge dynamics follows a self-similar solution at leading order, but, depending on the large-scale boundary conditions on the drop, significant corrections may arise to this solution. This dynamics is studied in detail using numerical simulations and through matched asymptotics. We show that the liquid lens coalescence can involve a global translation of the drops, a feature that is confirmed experimentally.