Continuous-Flow Sonocrystallization in Droplet-Based Microfluidics

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.