Simultaneous extraction and dissolution in surface nanodroplets for controlling the crystallization of small organic...

Droplets have been widely used for studying crystallization reactions which are difficult to control in the bulk. Most crystallization studies, however, rely on the evaporation or dissolution of the droplet liquid to achieve crystal growth, but it is difficult to control the crystallization process due to the uneven evaporation flux around the droplet. Here we show a new method for inducing and controlling the crystallization of organic molecules from an unsaturated solution using surface nanodroplets. Our method achieves crystallization by inducing liquid-liquid extraction of the organic molecules from its aqueous solution into surface nanodroplets. Concurrently, the surface nanodroplets slowly dissolve due to the continuous flowing aqueous stream. The synergy between liquid-liquid extraction and droplet dissolution drives supersaturation inside the surface nanodroplet that ultimately leads to nucleation and crystallization of organic molecules. The influence of molecule concentration and solution flow rate is investigated by tuning these parameters independently. Using trimesic acid (TMA) as model compound, we show that faster crystallization is achieved at higher TMA concentration, attributable to the higher driving force for diffusion of TMA into the surface nanodroplets. Additionally, thinner crystals are observed at higher solution flow rates, mainly due to a thinner concentration boundary layer around the surface nanodroplet which enables faster transport of TMA into the droplet. The method shown in this work can potentially be used in the formation of single crystals which is important in many areas including semiconductors, pharmaceuticals, and energy devices.