Coalescence of surfactant-laden droplets

TL;DR

This study uses molecular dynamics simulations to reveal how surfactants alter droplet coalescence, showing significant differences from pure water droplets in bridge growth dynamics, collective excitations, and coalescence rates, with implications for various technologies.

Contribution

It uncovers the surfactant mass-transport mechanism and identifies key differences in coalescence behavior compared to pure water droplets through detailed simulations.

Findings

Slower coalescence rate with increased surfactant concentration

Absence of multiple thermally excited precursors in surfactant-laden droplets

Presence of an initial thermal and inertial power-law regime in bridge growth

Abstract

Droplet coalescence is an important process in nature and various technologies (e.g. inkjet printing). Here, we unveil the surfactant mass-transport mechanism and report on several major differences in the coalescence of surfactant-laden droplets as compared to pure water droplets by means of molecular dynamics simulation of a coarse-grained model. Large scale changes to bridge growth dynamics are identified, such as the lack of multiple thermally excited precursors, attenuated collective excitations after contact, slowing down in the inertial regime due to aggregate-induced rigidity and reduced water flow, and a slowing down in the coalescence rate (deceleration) when surfactant concentration increases, while at the same time we also confirm the existence of an initial thermal, and a power-law, inertial, regime of the bridge growth dynamics in both the pure and the surfactant-laden droplets. Thus, we unveil the key mechanisms in one of the fundamental topological processes of liquid droplets containing surfactant, which is crucial in relevant technologies.