title: Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop.
abstract: Evaporating liquid droplets are omnipresent in nature and technology, such as in inkjet printing, coating, deposition of materials, medical diagnostics, agriculture, the food industry, cosmetics, or spills of liquids. Whereas the evaporation of pure liquids, liquids with dispersed particles, or even liquid mixtures has intensively been studied over the past 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 a 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 because of 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 that typifies the so-called "Ouzo effect." Once all ethanol has evaporated, the drop, which now has a characteristic nonspherical 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, all water has evaporated, leaving behind a tiny spherical cap-shaped oil drop.
authors.affiliation: Physics of Fluids Group, Department of Science and Technology, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: Huanshu
authors.initials: H
authors.lastname: Tan
authors.affiliation: Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands;
authors.forename: Christian
authors.initials: C
authors.lastname: Diddens
authors.affiliation: Physics of Fluids Group, Department of Science and Technology, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: Pengyu
authors.initials: P
authors.lastname: Lv
authors.affiliation: Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands; Department of Electrical Engineering, Mathematics, and Computer Science, University of Twente, 7500 AE Enschede, The Netherlands;
authors.forename: J G M
authors.initials: JG
authors.lastname: Kuerten
authors.affiliation: Soft Matter & Interfaces Group, School of Engineering, Royal Melbourne Institute of Technology University, Melbourne, VIC 3001, Australia; d.lohse@utwente.nl xuehua.zhang@rmit.edu.au.
authors.forename: Xuehua
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Physics of Fluids Group, Department of Science and Technology, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, 7500 AE Enschede, The Netherlands; Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany d.lohse@utwente.nl xuehua.zhang@rmit.edu.au.
authors.forename: Detlef
authors.initials: D
authors.lastname: Lohse
keywords: Ouzo effect; different volatilities; nucleation; sessile droplets; ternary droplet evaporation