Investigation of the Effect of Ultrasound Parameters on Continuous Sonocrystallization in a Millifluidic Device

Continuous-flow crystallization of adipic acid in a millichannel chip equipped with a piezoelectric element is presented and investigated experimentally and numerically. A single, straight channel chip (cross section: 2 mm X 5 mm, length: 76 mm) made of glass, which is ultrasonically transparent, was designed and fabricated. The piezoelectric element allows studying the effect of different ultrasound frequencies in the kHz to MHz range. Ultrasound was applied in burst mode to reduce heating; this allowed operating at higher levels of input power. To accurately control the temperature of the fluid, Peltier elements were used to cool the bottom and top surfaces of the chip. Crystallization was performed in isothermal conditions, ensuring that the temperature and in turn the supersaturation were kept uniform along the channel. The effect of ultrasound frequency and sonication time was studied. Crystal size distributions at different operating conditions were obtained by laser diffraction. The distributions were narrow, with coefficients of variation similar to 0.5, while the mean sizes were small (similar to 30 mu m) and decreased when the sonication time increased. The crystal production rate increased by increasing the sonication time; this suggests that ultrasound enhances nucleation. On the other hand, in crystal breakage experiments, no difference in the size distribution of the seed crystals entering and leaving the device was observed, and hence, in this setup, ultrasound does not cause breakage. Numerical simulations of wave propagation in aqueous solution were utilized to predict the probability of cavitation, adopting a suitable cavitation threshold. The simulations showed that high pressure amplitudes are achievable inside the channel at low frequencies. The size range of bubbles which undergo violent collapse at different pressure amplitudes and frequencies was quantified. By increasing the frequency in the simulations, it was observed that the probability of transient cavitation decreases. The theoretical prediction of negligible transient cavitation at higher frequencies, in conjunction with the absence of crystals at such frequencies, indicates a strong link between transient cavitation and sonocrystallization.