can be leveraged for speeding up chemical reactions, [14] concentrating chemicals from a sample based on partition coeffi cients. [15] Surface nanodroplets are femtoliter-scale droplets, with a height ranging from ≈5-1000 nm, formed on a target substrate. [16,17] The small volume and pinning effect around their boundary provide the stability of surface nanodroplets under the shear from the flow of an analyte or reactant solution, making surface nanodroplets suitable for rapid micro extraction [18] and droplet reactions [19] in continuous flowin systems. Recent advances in machine learning based methods may lead to rapid and automated droplet generation to establish training data for understanding dynamics of droplets consisting of complex media, [20] reaction kinetics in droplets, [21] and droplet based quantification and determination of chemical species. [22] In a variety of applications, the composition of multicomponent droplets is required to be optimized in order to achieve the desired functionality and performance of droplets. For example, in liquid-liquid extraction of chemical compounds from liquid solutions, binary droplets consisting of two solvents are used for the extraction performance which depends on the droplet composition because the solubility of a compound is different for every extractant. Binary organic droplets can be excellent media for efficient extraction of organic pollutants or drug compounds with different polarity from aqueous media at once. [23][24][25] Zhu et al. [23] used binary droplets of methylene chloride and n-hexane to extract and detect polycyclic aromatic hydrocarbons from environmental water samples down to a concentration of 2.1 ng L 1 − . Wang et al. [24] reported that the extraction efficiency of nicotine and cotinidine from urine sample was maximized when undecanol-chloroform binary droplets were used at a volumetric ratio of 1:1 with average recoveries of 91.5% and 79% for nicotine and cotinine, respectively. On the other hand, when only undecanol was used, the extraction efficiency for cotinine was less than 10% while the efficiency for nicotine reached 90%. This shows the strong influence of extractant type on the extraction performance. Thus, reliable formation of multicomponent droplets with optimized droplet composition is of high importance.Evaporation of deposited drops may be a simple approach leading to formation of nanodroplets on surfaces. However, different compounds in a droplet may lead to selective evaporation of more volatile liquid and complicated transport phenomena due to gradients in surface tension or chemical
