Adipic acid primary nucleation kinetics from probability distributions in droplet-based systems under stagnant and flow conditions

In this work, we present a microfluidic approach that allows performing nucleation studies under different fluid dynamic conditions. We determine primary nucleation rates and nucleation kinetic parameters for adipic acid solutions by using liquid/liquid segmented flow in capillary tubes in which the crystallizing medium is partitioned into small droplets. We do so by measuring the probability of crystal presence within individual droplets under stagnant (motionless droplets) and flow (moving droplets) conditions as a function of time, droplet volume, and supersaturation. Comparing the results of the experiments with the predictions of the classical nucleation theory model and of the mononuclear nucleation mechanism model, we conclude that adipic acid nucleates mainly via a heterogeneous mechanism under both fluid dynamic conditions. Furthermore, we show that the flow conditions enhance the primary nucleation rate by increasing the kinetic parameters of the process without affecting the thermodynamic parameters. In this regard, a possible mechanism is discussed on the basis of the enhancement of the attachment frequency of nucleation caused by the internal recirculation that occurs within moving droplets.