Synergistic effects of the alternating application of low and high frequency ultrasound for particle synthesis in microreactors

Ultrasound (US) is a promising method to address clogging and mixing issues in microreactors (MR). So far, low frequency US (LFUS), pulsed LFUS and high frequency US (HFUS) have been used independently in MR for particle synthesis to achieve narrow particle size distributions (PSD). In this work, we critically assess the advantages and disadvantages of each US application method for the case study of calcium carbonate synthesis in an ultrasonic microreactor (USMR) setup operating at both LFUS (61.7 kHz, 8 W) and HFUS (1.24 MHz, 1.6 W). Furthermore, we have developed a novel approach to switch between LFUS and HFUS in an alternating manner, allowing us to quantify the synergistic effect of performing particle synthesis under two different US conditions. The reactor was fabricated by gluing a piezoelectric plate transducer to a silicon microfluidic chip. The results show that independently applying HFUS and LFUS produces a narrower PSD compared to silent conditions. However, at lower flow rates HFUS leads to agglomerate formation, while the reaction conversion is not enhanced due to weak mixing effects. LFUS on the other hand eliminates particle agglomerates and increases the conversion due to the strong cavitation effect. However, the required larger power input leads to a steep temperature rise in the reactor and the risk of reactor damage for long-term operation. While pulsed LFUS reduces the temperature rise, this application mode leads again to the formation of particle agglomerates, especially at low LFUS percentage. The proposed application mode of switching between LFUS and HFUS is proven to combine the advantages of both LFUS and HFUS, and results in particles with a unimodal narrow PSD (one order of magnitude reduction in the average size and span compared to silent conditions) and negligible rise of the reactor temperature.