Time-Resolved Dynamic Crystallization at Liquid/Vapor Interface

We report the behavior of nanodroplets containing aqueous NaOH solutions of different concentrations, visualized using transmission electron microscopy (TEM). Under e-beam irradiation, highly charged aqueous NaOH droplets dewet with edge crystallization before erupting into satellite nanodroplets. For the first time, we observe dynamic nanocrystal precipitation from nanodroplet followed by redissolution in a periodic manner. Time-resolved images show that the crystalline precipitates occur predominantly in two distinct geometric shapes-hexagonal and square. Through structural, elemental, and morphological analyses, we deduce that the crystalline precipitate is monohydrate NaOH·H₂O, whose orthorhombic structure forms the underlying basis for the evolved crystalline shapes. Through numerical simulations, we show how the observed preferential orientation of the crystal precipitate depends on crystal morphology and relative droplet-crystal size, but not on electrical forces, which govern droplet deformations. Together, our results reveal strong dynamics at the interface, including ionic chemistry, nanoscale physics, and hydrodynamics.