GALCIT Colloquium

Nanodroplets on a solid surface (i.e. surface nanodroplets or sessile nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, lab-on-chip devices, and near-field imaging techniques. Oil nanodroplets can be produced on a solid-liquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. We report on our experimental and theoretical investigation of the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We found significant effects from the flow rate and the flow geometry on the droplet size. We have developed a theoretical framework to account for these effects. The main idea is that the droplet nuclei are exposed to an oil oversaturation pulse during the exchange process. 

We will also report on surface nanodroplet formation on patterned surfaces of all kinds, which changes the growth or shrinkage mode of the nanodroplets. In particular, we will show how nanodroplets nucleate at the rim of spherical cap microstructures on a substrate, due to a pulse of oversaturation supplied by a solvent exchange process. We find that, while growing at the rim of the microcap, the nanodroplets self-organize into highly symmetric arrangements, with respect to position, size, and mutual distance.

Next, we will show that gravity can have an effect of the formation of surface droplets. Counter-intuitively, it can also have an effect on the dissolution of small immersed sessile droplets. We study the dissolution process of long-chain alcohol (of various types) sessile droplets in water is studied, disentangling diffusive and convective contributions. The latter can arise for high solubilities of the alcohol, as the density of the alcohol-water mixture is then considerably less than that of pure water, giving rise to buoyancy driven convection. 

Finally, we will report on our experimental, theoretical, and numerical results on the evaporation of droplets of ternary mixtures such as Ouzo (as known from daily life—a transparent mixture of water, ethanol, and anise oil). The evaporation process can trigger a phase transition and the nucleation of microdroplets of one of the components of the mixture. We find and explain four life phases of the evaporating ouzo droplet.