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A novel design for continuous flow sonocrystallization of adipic acid in a capillary device is presented and investigated experimentally and numerically. The effect of supersaturation and ultrasound power is studied. To elucidate the relationship between crystallization and cavitation, sonochemiluminescence and sonoemulsification experiments are performed, and numerical investigation of the wave propagation in aqueous solution is used to predict the probability of cavitation. Crystal size distribution at different operating conditions is obtained by laser diffraction. Narrow size distributions, small mean size of crystals (ca. 15 μm), and high crystal production rate are achieved when applying ultrasound. In addition, numerical simulations of pressure distribution show that high pressure amplitudes are obtainable near the vicinity of the sonoprobe tip. Using a cavitation threshold formulation, the distance from the tip where transient cavitation takes place is quantified. The results are in agreement with the experimental findings, in which by increasing the distance between capillary and sonoprobe, emulsification, sonochemiluminescence, and nucleation decrease. It is concluded that transient cavitation of bubbles is a significant mechanism for enhancing nucleation of crystals among the several proposed in the literature.

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.