Interactions between droplets in immiscible liquid suspensions and the influence of surfactants

TL;DR

This paper develops a density functional theory-based method to analyze droplet interactions in immiscible liquids, demonstrating how surfactants like alcohol can stabilize oil droplets by altering interfacial tension and interaction potentials.

Contribution

The authors introduce a general theoretical framework to compute effective droplet interactions and show how surfactants can induce stability by modifying interfacial properties.

Findings

Alcohol reduces oil-water interfacial tension.

Surfactant effects can lead to repulsive interactions between droplets.

Droplets can be stabilized with a free-energy barrier preventing coalescence.

Abstract

We develop a general method for determining the effective interaction potential between two or more droplets suspended within a fluid phase. Our approach is based on classical density functional theory. Here, we apply the method to determine the interaction potential between oil droplets suspended in water and also consider the influence of adding a third species, alcohol. This ternary mixture is that found in the ouzo beverage. The ouzo system exhibits spontaneous emulsification when the neat spirit is mixed with water. The oil emulsion that forms has been observed to be surprisingly long-lived. Here we show that the alcohol in the system does indeed play a role in making the droplets more stable, by decreasing the oil-water interfacial tension and therefore also the strength of the attractive interactions between droplets. Within our theory, the surfactant nature of the alcohol can be enhanced without changing the bulk fluid thermodynamics. In fact, our theory can be used to model surfactant mixtures. In this model, the effective interaction between pairs of oil droplets can become repulsive, with a free-energy barrier to droplets merging, thus making them stable.