# Interactions between Droplets in Immiscible Liquid Suspensions and the Influence of Surfactants

## TL;DR

This paper studies how oil droplets interact in water-alcohol mixtures, like in ouzo, and how alcohol stabilizes the droplets by reducing their attraction.

## Contribution

A new method using density functional theory to model droplet interactions and surfactant effects in ternary mixtures is introduced.

## Key findings

- Alcohol reduces oil-water interfacial tension, stabilizing oil droplets in water.
- The model shows alcohol can make droplet interactions repulsive, preventing merging.
- The theory can be extended to model surfactant mixtures without altering bulk thermodynamics.

## 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.