In this work, we use an electrodynamic balance (EDB) to study primary crystal nucleation from single levitated aqueous potassium nitrate (KNO3) solution droplets under isothermal conditions. We investigate crystallization in droplets with volumes less than one nanoliter. From induction time distributions we derive nucleation rates of KNO3. Nucleation processes were found to occur at different mechanisms. Results are interpreted based on the classical nucleation theory (CNT) to gain more information about the prevailing nucleation mechanism. We also investigate the shape and morphology of crystals using scanning electron microscopy (SEM). Two typical morphology types of crystallized particles could be identified, depending on whether nucleation occurred during the evaporation phase or at constant supersaturation of solute.

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Crystal nucleation is important to control the product properties in industrial crystallization processes. To investigate crystallization phenomena, methods which rely on microscopic volumes have gained relevance over the last decade. Microfluidic devices are suitable for carrying out crystallization experiments based on a large set of individual droplets in the nanoliter range. In this work, we propose a simple method to manufacture such devices from polycarbonate as an alternative to conventional chips made of poly (dimethylsiloxane). The microfluidic device consists of two main functional parts: A T-junction for droplet generation and a section for storage and observation of up to 400 individual droplets. Using these manufactured devices, it is easy to produce and store highly monodisperse droplets of substances that require either a hydrophilic or hydrophobic surface of the microchannel. Since crystal nucleation is a stochastic process which depends on the sample volume, a reproducible droplet volume is of great importance for crystallization experiments. The versatile applicability of the manufactured devices is demonstrated for substances which are used in different crystallization applications, for example, solution crystallization (aqueous potassium nitrate solution) and melt crystallization (ethylene glycol distearate). Finally, we demonstrate that the manufactured microfluidic devices in our experimental setup can be used to conduct crystal nucleation measurements. Based on these measurements we discuss our results with respect to state-of-the-art nucleation models.