Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking

Abstract

The Greek aperitif Ouzo is not only famous for its specific anise-flavored taste, but also for its ability to turn from a transparent miscible liquid to a milky-white colored emulsion when water is added. Recently, it has been shown that this so-called Ouzo effect, i.e. the spontaneous emulsification of oil microdroplets, can also be triggered by the preferential evaporation of ethanol in an evaporating sessile Ouzo drop, leading to an amazingly rich drying process with multiple phase transitions [H. Tan et al., Proc. Natl. Acad. Sci. USA 113(31) (2016) 8642]. Due to the enhanced evaporation near the contact line, the nucleation of oil droplets starts at the rim which results in an oil ring encircling the drop. Furthermore, the oil droplets are advected through the Ouzo drop by a fast solutal Marangoni flow.In this article, we investigate the evaporation of mixture droplets in more detail, by successively increasing the mixture complexity from pure water over a binary waterethanol mixture to the ternary Ouzo mixture (water, ethanol and anise oil). In particular, axisymmetric and full three-dimensional finite element method simulations have been performed on these droplets to discuss thermal effects and the complicated flow in the droplet driven by an interplay of preferential evaporation, evaporative cooling and solutal and thermal Marangoni flow. By using image analysis techniques and micro-PIV measurements, we are able to compare the numerically predicted volume evolutions and velocity fields with experimental data. The Ouzo droplet is furthermore investigated by confocal microscopy. It is shown that the oil ring predominantly emerges due to coalescence.

Convection-dominated dissolution for single and multiple immersed sessile droplets

“…For a multi-component drop dissolving in a host liquid, there is formation of Marangoni flow caused by the variation of surface tension over the droplet surface, which in addition can influence the behaviour of emulsification (Tan et al 2019). Similarly, the Marangoni flow also plays a crucial role in multi-component sessile Convection-dominated dissolution for immersed sessile drops 892 A21-3 droplet evaporation (Scriven & Sternling 1960;Tan et al 2016;Diddens et al 2017;Kim et al 2017;Edwards et al 2018;Li et al 2018bLi et al , 2019.…”

The lifetimes of evaporating sessile droplets are significantly extended by strong thermal effects

“…However, all of these works use the basic diffusion-limited model for droplet evaporation (see, for example, Popov 2005) which does not account for the variation of the saturation concentration of the vapour with temperature. As a result, none of these works accounts for the influence of the thermal properties of the system on the evaporation of the droplets, which can be significant (see, for example, Dunn et al 2008, 2009a,b, Sefiane & Bennacer 2011, Sobac & Brutin 2012, Ait Saada, Chikh and Tadrist 2013, Lopes et al 2013and Diddens et al 2017. The aim of the present work is to gain further insight into this issue by using an extended model to obtain asymptotic solutions for the evolution, and hence explicit expressions for the lifetimes, of droplets when the influence of the thermal properties of the system is strong.…”

Gravitational Effect in Evaporating Binary Microdroplets

“…To validate the experimental results, corresponding finite element simulations were performed. To that end, an axisymmetric model [43] was employed, which had already been successfully validated with binary waterethanol and ternary ouzo droplets [48]. The model solves the diffusion equation for the water vapor concentration C w,air in the gas phase, assuming vapor-liquid equilibrium according to Raoult's law.…”