Nanodroplets on a solid surface (i.e., surface nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, laboratory-on-chip devices, and near-field imaging techniques. Oil nanodroplets can be produced on a solidliquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. In this work, we experimentally and theoretically investigate the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We find significant effects from the flow rate and the flow geometry on the droplet size. We develop 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. The analysis shows that the volume of the nanodroplets increases with the Peclet number Pe of the flow as ∝ Pe 3=4 , which is in good agreement with our experimental results. In addition, at fixed flow rate and thus fixed Peclet number, larger and less homogeneously distributed droplets formed at less-narrow channels, due to convection effects originating from the density difference between the two solutions of the solvent exchange. The understanding from this work provides valuable guidelines for producing surface nanodroplets with desired sizes by controlling the flow conditions. N anoscale droplets on a substrate (1) are an essential element for a wide range of applications, namely laboratory-on-chip devices, simple and highly efficient miniaturized reactors for concentrating products, high-throughput single-bacteria or singlebiomolecular analysis, encapsulation, and high-resolution imaging techniques, among others (2-5). These droplets are of great interest also because they can have a payload and can flow internally in response to external flow. As a consequence, such droplets are widely exploited in formulation industries. Quite some effort has been devoted to produce a large amount of nanodroplets in a controlled way. The current techniques include trapping by microcavities, emulsion direct adsorption, microprinting, and others (6). The solvent exchange process is a simple and generic approach for producing droplets or bubbles at solid-liquid interfaces that are only several tens to hundreds of nanometers in height, or a few femtoliters in volume (7-11). The approach has attractive advantages, such as its capability of producing a large number of nanodroplets in one simple step, and its generality in chemical composition of the droplet liquid, and flexibility in aspect ratio of the droplets and spatial structure or size of the substrate (9, 12).For the formation of surface nanodroplets by solvent exchange, a hydrophobic substrate is exposed sequentially to two miscible solutions of oil, where the second solvent has a lower solubility of oil than the first. Such solubility difference leads to supersaturation of the liquid with oil during the solvent exchange and consequently to the nucleation of nanodroplets on...
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. What happens when an Ouzo drop is evaporating? The Greek drink Ouzo (or the French Pastis or the Turkish Raki) consists of an optically transparent ternary mixture of water, ethanol, and anise oil. When served, water is often added, leading to the nucleation of many tiny oil droplets, which give the drink its milky appearance. This is the so-called Ouzo effect (19). As we will see in this paper, this problem can also become paradigmatic because of its extremely rich behavior, now for the evaporation-triggered phase separation of ternary liquids and droplet nucleation therein.The reason for the Ouzo effect lies in the varying solubility of oil in ethanol-water mixtures: With increasing water concentration during the solvent exchange (i.e., water being added), the oil solubility decreases, leading to droplet nucleation in the bulk and-if present-also on hydrophobic surfaces (so-called surface nanodroplets) (20, 21).
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.
The assembly of colloidal particles from evaporating suspension drops is seen as a versatile route for the fabrication of supraparticles for various applications. However, drop contact line pining leads to uncontrolled shapes of the emerging supraparticles, hindering this technique. Here we report how the pinning problem can be overcome by self-lubrication. The colloidal particles are dispersed in ternary drops (water, ethanol, and anise-oil). As the ethanol evaporates, oil microdroplets form (‘ouzo effect’). The oil microdroplets coalesce and form an oil ring at the contact line, levitating the evaporating colloidal drop (‘self-lubrication’). Then the water evaporates, leaving behind a porous supraparticle, which easily detaches from the surface. The dispersed oil microdroplets act as templates, leading to multi-scale, fractal-like structures inside the supraparticle. Employing this method, we could produce a large number of supraparticles with tunable shapes and high porosity on hydrophobic surfaces.
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