Effect of acoustic streaming on continuous flow sonocrystallization in millifluidic channels

This work is concerned with the effect of ultrasound on mixing and consequently on crystallization in millifluidic channels. An ultrasonic horn with frequency of 20 kHz was placed inside a vessel with a well-defined geometry containing a single capillary tube with internal diameter (I.D.) of 1.55 or 3.2 mm operated with a fluid flow rate between 1 and 17.6 ml/min. This system was employed to produce crystals of adipic acid in the range of 15-35 mu m. The effect of ultrasound on flow patterns, residence time distribution (RTD) and cooling crystallization were investigated experimentally and numerically. Simulations of acoustic and velocity fields inside the millichannel acting as crystallizer allowed characterizing the acoustic streaming generated within it. In the large capillary, especially at small flow rates, acoustic streaming influenced the flow field, inducing vortices and leading to significant changes in RTD. Conversely, in the small capillary, ultrasound affected the flow field and the RTD negligibly, and a laminar velocity profile with straight streamlines was obtained. As a consequence, different crystallization behaviours were observed in the two capillaries; in particular, while the mean crystal size increased with the sonication residence time in the 1.55 mm I.D. capillary, it decreased in the 3.2 mm I.D. capillary. This difference highlights the importance of considering acoustic streaming when designing sono-crystallizers.