Influence of the time-dependent surfactant adsorption on the lifetime of a drop pressed by buoyancy against a planar interface

TL;DR

This study uses emulsion stability simulations to analyze how time-dependent surfactant adsorption affects the coalescence lifetime of a buoyant oil droplet at a planar water/oil interface, highlighting the role of surfactant diffusion.

Contribution

It introduces a model linking surfactant diffusion dynamics to droplet coalescence times, providing new insights into emulsion stability mechanisms.

Findings

Coalescence time distributions vary with surfactant diffusion constant.

Higher apparent diffusion constants lead to longer coalescence times.

The model explains the origin of different coalescence time distributions.

Abstract

Emulsion Stability Simulations (ESS) of deformable droplets are used to study the influence of the time-dependent adsorption on the coalescence time of a 200-$\mu$m drop of soybean oil pressed by buoyancy against a planar water/oil interface. The interface is represented by a 5000-$\mu$m drop of oil fixed in the space. The movement of the small drop is determined by the interaction forces between the drops, the buoyancy force, and its thermal interaction with the solvent. The interaction forces depend on the surface concentration of surfactant molecules at the oil/water interfaces. Assuming diffusion limited adsorption, the surface excess of the surfactant becomes a function of its apparent diffusion constant, $D_{app}$. Distinct probability distributions of the coalescence time are obtained depending on the magnitude of $D_{app}$. The origin and the significance of these distributions are discussed.