# Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop

## TL;DR

This study investigates the evaporation process of ternary liquid mixtures, specifically an Ouzo drop, revealing four distinct phases driven by phase transitions and microdroplet nucleation during evaporation.

## Contribution

It provides the first detailed analysis and theoretical explanation of the four life phases of an evaporating ternary mixture, highlighting phase transitions and microdroplet nucleation.

## Key findings

- Identified four distinct evaporation phases of an Ouzo drop.
- Demonstrated phase transition from transparent to milky and back.
- Theoretically explained the nucleation and phase inversion processes.

## Abstract

Evaporating liquid droplets are omnipresent in nature and technology, such as in inkjet printing, coating, deposition of materials, medical diagnostics, agriculture, food industry, cosmetics, or spills of liquids. While the evaporation of pure liquids, liquids with dispersed particles, or even liquid mixtures has intensively been studied over the last two decades, the evaporation of ternary mixtures of liquids with different volatilities and mutual solubilities has not yet been explored. Here we show that the evaporation of such ternary mixtures can trigger a phase transition and the nucleation of microdroplets of one of the components of the mixture. As model system we pick a sessile Ouzo droplet (as known from daily life - a transparent mixture of water, ethanol, and anise oil) and reveal and theoretically explain its four life phases: In phase I, the spherical cap-shaped droplet remains transparent, while the more volatile ethanol is evaporating, preferentially at the rim of the drop due to the singularity there. This leads to a local ethanol concentration reduction and correspondingly to oil droplet nucleation there. This is the beginning of phase II, in which oil microdroplets quickly nucleate in the whole drop, leading to its milky color which typifies the so-called 'Ouzo-effect'. Once all ethanol has evaporated, the drop, which now has a characteristic non-spherical-cap shape, has become clear again, with a water drop sitting on an oil-ring (phase III), finalizing the phase inversion. Finally, in phase IV, also all water has evaporated, leaving behind a tiny spherical cap-shaped oil drop.